How long was Kurukshetra War fought?

The war was fought for 18 long days.
The Kurukshetra War is a war described in the Indian epic Mahabharata as a conflict that arose from a dynastic succession struggle between two groups of cousins of an Indian kingdom called Kuru, the Kauravas and Pandavas, for the throne of Hastinapura. It involved a number of ancient kingdoms participating as allies of the rival groups. The narrative describes individual battles and deaths of various heroes  of both sides, military formations, war diplomacy, meetings and  discussions among the characters, and the weapons used. The chapters (parvas) dealing with the war (from chapter six to ten) are considered amongst the oldest in the entire Mahabharat.

Day 1
When the battle was commenced, Bhishma went through the Pandava army wreaking havoc wherever he went but Abhimanyu,  Arjuna's son, seeing this went straight at Bhishma, defeated his  bodyguards and directly attacked the commander of the Kaurava forces.  The Pandavas suffered numerous losses and were defeated at the end of  the first day. Virata's sons, Uttara and Sweta, were slain by Shalya and Bhishma. Krishna consoled the distraught Yudhishthira saying that eventually victory would be his.

Day 2
The second day of the war commenced with a confident Kaurava army  facing the Pandavas. Arjuna, realising that something needed to be done  quickly to reverse the Pandava losses, decided that he must try to kill  Bhishma. Krishna skillfully located Bhishma's chariot and steered Arjuna  toward him. Arjuna tried to engage Bhishma in a duel, but the Kaurava  soldiers placed around Bhishma to protect him attacked Arjuna to try to  prevent him from directly engaging Bhishma. Arjuna and Bhishma fought a  fierce battle that raged for hours. Drona and Dhristadyumna similarly engaged in a duel during which Drona broke Dhristadyumna's bow numerous times. Bhima intervened and rescued Dhristadyumna. Duryodhana sent the Kalinga forces to attack Bhima and most of them lost their lives at his hands.  Bhishma immediately came to relieve the battered Kalinga forces. Satyaki,  who was assisting Bhima, shot at Bhishma's charioteer and killed him.  Bhishma's horses, with no one to control them, bolted carrying Bhishma  away from the battlefield. The Kaurava army had suffered great losses at  the end of the second day.

Day 3
Arjuna Wijaya statue in Central Jakarta depicting Krishna and Arjuna riding a chario
On the third day, Bhishma arranged the Kaurava forces in the  formation of an eagle with himself leading from the front, while  Duryodhana's forces protected the rear. Bhishma wanted to be sure of  avoiding any mishap. The Pandavas countered this by using the crescent  formation with Bhima and Arjuna at the head of the right and the left horns, respectively.  The Kauravas concentrated their attack on Arjuna's position. Arjuna's  chariot was soon covered with arrows and javelins. Arjuna, with amazing  skill, built a fortification around his chariot with an unending stream  of arrows from his bow. Abhimanyu and Satyaki combined to defeat the Gandhara forces of Shakuni. Bhima and his son Ghatotkacha attacked Duryodhana in the rear. Bhima's arrows hit Duryodhana, who  swooned in his chariot. His charioteer immediately drove them out of  danger. Duryodhana's forces, however, saw their leader fleeing the  battlefield and soon scattered. Bhishma soon restored order and  Duryodhana returned to lead the army. He was angry at Bhishma, however,  at what he saw as leniency towards the five Pandava brothers and spoke  harshly at his commander. Bhishma, stung by this unfair charge, fell on  the Pandava army with renewed vigor. It was as if there were more than  one Bhishma on the field.
Arjuna attacked Bhishma trying to restore order. Arjuna and Bhishma  again engaged in a fierce duel, however Arjuna's heart was not in the  battle as he did not like the idea of attacking his grand-uncle. During  the battle, Bhishma killed numerous soldiers of Arjuna's armies.

Day 4
The fourth-day of the battle was noted for the valour shown by Bhima.  Bhishma commanded the Kaurava army to move on the offensive from the  outset. While Abhimanyu was still in his mother's womb, Arjuna had  taught Abhimanyu on how to break and enter the chakra vyuha. But, before  explaining how to exit the chakra Vyuha, Arjuna was interrupted by  Krishna (other story: Abhimanyu's mother falls asleep while Arjuna was  explaining her). Thus from birth, Abhimanyu only knew how to enter the  Chakra vyuha but didn't know how to come out of it. When the Kauravas  formed the chakravyuha, Abhimanyu entered it but was surrounded and  attacked by a number of Kaurava princes. Arjuna joined the fray in aid  of Abhimanyu. Bhima appeared on the scene with his mace aloft and  started attacking the Kauravas. Duryodhana sent a huge force of  elephants at Bhima. When Bhima saw the mass of elephants approaching, he  got down from his chariot and attacked them single handedly with his  iron mace. They scattered and stampeded into the Kaurava forces killing  many. Duryodhana ordered an all-out attack on Bhima. Bhima withstood all  that was thrown at him and attacked Duryodhana's brothers, killing  eight of them. Bhima was soon struck by an arrow on the chest and sat  down in his chariot dazed.
Duryodhana was distraught at the loss of his brothers. Duryodhana,  overwhelmed by sorrow at the loss of his brothers, went to Bhishma at  the end of the fourth day of the battle and asked his commander how  could the Pandavas, facing a superior force against them, still prevail  and win. Bhishma replied that the Pandavas had justice on their side and  advised Duryodhana to seek peace.

Days 5–9
When the battle resumed on the fifth day, the slaughter continued.  The Pandava army again suffered against Bhishma's attacks. Satyaki bore  the brunt of Drona's  attacks and soon could not withstand them. Bhima drove by and rescued  Satyaki. Arjuna fought and killed thousands of soldiers sent by  Duryodhana to attack him. The unimaginable carnage continued during the  ensuing days of the battle.
The sixth day was marked by a prodigious slaughter. Drona caused immeasurable loss of life on the Pandava side. The formations of  both the armies were broken. On day 7 Drona slays Shankya, son of  Virata. On the eighth day Bhima killed eight of Dhritarashtra's sons.  Iravan, the son of Arjuna and Ulupi kills 5 brothers of Shakuni/princes  of Gandhara. Duryodhana sends Alamvush to kill Iravan and the latter is killed by the rakshasa after a brave fight.
On the ninth day Krishna, overwhelmed by anger at the apparent  inability of Arjuna to defeat Bhishma, rushed towards the Kaurava  commander, the wheel of a fallen chariot in his hands. Bhishma lays down  his arms and is ready to die at the hands of the Lord, but Arjuna stops  him, reminding of his promise not to wield a weapon. Realizing that the  war could not be won as long as Bhishma was standing, Krishna suggested  the strategy of placing a eunuch in the field to face him. Some sources  however state that it was Yudhishthira who visits Bhishma's camp at  night asking him for help. To this Bhishma says that he would not fight a  eunuch.

Day 10
Bhishma on a deathbed of arrows, from a collection of the Smithsonian Institution
On the tenth day, the Pandavas, unable to withstand Bhishma's prowess, decided to put Shikhandi,  who had been a woman in a prior life in front of Bhishma, as Bhishma  has taken a vow not to attack a woman. Shikhandi's arrows fell on  Bhishma without hindrance. Arjuna positioned himself behind Shikhandi,  protecting himself from Bhishma's attack and aimed his arrows at the  weak points in Bhishma's armour. Soon, with arrows sticking from every  part of his body, the great warrior fell from his chariot. His body did  not touch the ground as it was held aloft by the arrows protruding from  his body.
The Kauravas and Pandavas gathered around Bhishma and at his request,  Arjuna placed three arrows under Bhishma's head to support it. Bhishma  had promised his father, King Shantanu, that he would live until  Hastinapur were secured from all directions. To keep this promise,  Bhishma used the boon of "Ichcha Mrityu"(self wished death) given to him  by his father. After the war was over, when Hastinapur had become safe  from all sides and after giving lessons on politics and Vishnu Sahasranama to the Pandavas, Bhishma died on the first day of Uttarayana.

Day 11
With Bhishma unable to continue, Karna entered the battlefield, much to Duryodhana's joy. He made Drona the supreme commander of the Kaurava forces. Karna and Duryodhana  wanted to capture Yudhishthira alive. Killing Yudhishthira in battle  would only enrage the Pandavas more, whereas holding him as hostage  would be strategically useful. Drona formulated his battle plans for the  eleventh day to this aim. He cut down Yudhishthira's bow and the  Pandava army feared that their leader would be taken prisoner. Arjuna  rushed to the scene, however and with a flood of arrows made Drona  retreat.

Day 12
With his attempts to capture Yudhishthira thwarted, Drona confided to Duryodhana that it would be difficult as long as Arjuna was around. He summoned King Bhagadatta, the monarch of Pragjyotisha.  Bhagadatta had thousands of gigantic elephants in his stable and was  considered the strongest warrior on this planet in elephant warfare.  Bhagadatta attacked Arjuna with his gigantic elephant named Suprateeka.  It was a fierce battle in which Bhagadatta matched Arjuna astra for astra. Drona continued to try and capture Yudhishthira. The  Pandavas however fought hard and delivered severe blows to the Kaurava  army.

Day 13
The King of Trigartadesa, Susharma along with his 3 brothers who were fighting on the Kaurava side made a  pact that they would make sure that Arjuna comes and doesn't break the  Chakra Vuyh and kill him or die. They went into the battlefield on the  twelfth day and challenged Arjuna. Arjuna gave them a fierce fight in  which the brothers fell dead after fighting a brave fight. Drona  continued to try and capture Yudhishthira.
On the other side of the battlefield, the remaining four Pandavas and  their allies were finding it impossible to break Dronacharya's "Chakra  Vyuh" formation. As Arjuna was busy fighting with the Trigartadesa  princes and the Prajayogastha monarch on the other side of the  battlefield, he could not be summoned to break the Chakra Vyuha formation, which could only be broken by entering and exiting the formation. Yudhishthira instructed, Abhimanyu,  one of Arjuna's sons to break the Chakra vyuha formation. Abhimanyu  knew the secret of entering the Chakra vyuh formation, but did not know  how to exit it. Abhimanyu slew tens of thousands of warriors. Some of them included Vrihadvala, the ruler of Kosala, the ruler of Kratha, the ruler of Assaka,  Martikavat prince of Bhojas, Shalya's son Rukmartha, Shalya's younger  brother and so many others. He also killed Duryodhana's son. Duryodhana  got enraged and ordered Durmashana, who was Dushasana's son to attack Abhimanyu but he died under the hands of Abhimanyu. Next he ordered his men to attack Abhimanyu all at once. Abhimanyu fought but was surrounded and killed by many warriors in a combined attack.
Upon learning of the death of his son, Arjuna vowed to kill Jayadratha on the morrow before the battle ended at sunset, otherwise he would throw himself into the fire.

Day 14
While searching for Jayadrath on the battlefield, Arjuna slew an akshauhini (battle formation that consisted of 21,870 chariots (Sanskrit ratha);  21,870 elephants; 65,610 cavalry and 109,350 infantry) of Kaurav  soldiers. The Shakatavuyha Kaurav army tightly protected Jayadratha,  however, preventing Arjuna from attacking him. Finally, in late  afternoon, Arjuna found Jayadratha guarded by the mighty Kaurav army.  Seeing his friend's plight, Lord Krishna raised his Sudarshan Chakra to cover the sun, faking a sunset. Arjun fought a powerful battle with  Jayadrath and finally defeated him. Then, Arjuna shot a powerful arrow  decapitating Jayadrath. While Arjuna destroying the rest of the  Shakatavuyha, Vikarna, the third eldest Kaurava, challenged Arjuna to an  archery fight. Arjuna asks Bhima to decimate Vikarna, but Bhima refused  to, because Vikarna defended the Pandavas during the Draupadi Vastrapaharanam.  Bhima and Vikarna showered arrows at each other. Later Bhima throws his  mace at Vikarna, killing him. The muscular Pandava was devastated and  mourned his death saying he was a man of Dharma and it was a pity how he  lived his life. Drona kills Vrihatkshatra, the ruler of Kekaya and Dhrishtakethu, the ruler of Chedi. Arjuna kills Vrishasena, the son of Karna.
The battle continued past sunset. When the bright moon rose, Ghatotkacha, the son of Bhima slaughtered numerous warriors, attacking while flying in the air. Karna  stood against him and both fought fiercely until Karna released the  Shakti, a divine weapon given to him by Indra. Ghatotkacha increased his size and fell dead on the Kaurav army killing an akshouhini of them.

Day 15
After King Drupada and King Virata were slain by Drona, Bhima and Dhristadyumna fought him on the fifteenth day. Because Drona was very powerful and inconquerable having the irresistible Brahmanda astra,  Krishna hinted to Yudhishthira that Drona would give up his arms if his  son Ashwatthama was dead. Bhima proceeded to kill an elephant named  Ashwatthama and loudly proclaimed that Ashwatthama was dead. Drona  approached Yudhishthira to seek the truth of his son's death.  Yudhishthira proclaimed Ashwathama Hatahath, Naro Va Kunjaro Va,  implying Ashwathama had died but he was nor sure whether it was a  Drona's son or an elephant, The latter part of his proclamation (Naro va  Kunjaro va) were drowned out by sound of the conch blown by Krishna  intentionally (a different version of the story is that Yudhishthira  pronounced the last words so feebly that Drona could not hear the word  elephant). Prior to this incident, the chariot of Yudhishthira,  proclaimed as Dharma raja (King of righteousness), hovered a few inches off the ground. After the event, the chariot landed on the ground as he lied.
Drona was disheartened, and laid down his weapons. He was then killed by Dhristadyumna to avenge his father's death and satisfy his vow. Later, the Pandava's mother Kunti secretly met her abandoned son Karna and requested him to spare the Pandavas, as they were his younger  brothers. Karna promised Kunti that he would spare them except for  Arjuna, but also added that he would not fire a same weapon against  Arjun twice.

Day 16
Bhima fulfilling his promise regarding Duhshasan in the Mahabharata field
On the sixteenth day, Karna was made the supreme commander of the Kuru army. Karna fought valiantly  but was surrounded and attacked by Pandava generals, who were unable to  prevail upon him. Karna inflicted heavy damage on the Pandava army,  which fled. Then Arjuna successfully resisted Karna's weapons with his  own and also inflicted casualties upon the Kaurava army. The sun soon  set and with darkness and dust making the assessment of proceedings  difficult, the Kaurava army retreated for the day.
On the same day, Bhima swung his mace and shattered Dushasana's  chariot. Bhima seized Dushasana, ripped his right hand from shoulder and  killed him, tearing open his chest and drinking his blood and carrying  some to smear on Draupadi's untied hair, thus fulfilling his vow made  when Draupadi was humiliated.

Day 17
Karna(right) confronts Arjuna, who will later kill Karna, in the Kurukshetra war
On the seventeenth day, Karna defeated the Pandava brothers Nakula, Bhima, sahadeva and Yudhishthira in battle but spared their lives. Later, Karna resumed duelling with Arjuna.  During their duel, Karna's chariot wheel got stuck in the mud and Karna  asked for a pause. Krishna reminded Arjuna about Karna's ruthlessness  unto Abhimanyu while he was similarly left without chariot and weapons. Hearing his  son's fate, Arjuna shot his arrow and decapitated Karna. Before the  day's battle, Karna's sacred armour ('Kavacha') and earrings ('Kundala')  were taken as alms by Lord Indra when asked for, which resulted in his  death by Arjuna's arrows.

Day 18
On the 18th day, Shalya took over as the commander-in-chief of the remaining Kaurava forces. Yudhishthira killed king Shalya in a spear combat and Sahadeva killed Shakuni. Realizing that he had been defeated, Duryodhana fled the battlefield and took refuge in the lake, where the Pandavas  caught up with him. Under the supervision of the now returned Balarama, a mace battle took place between Bhima and Duryodhana. Bhima flouted the rules (under instructions from Krishna) to strike Duryodhana beneath the waist in which he was mortally wounded.
Ashwatthama, Kripacharya, and Kritavarma met Duryodhana at his deathbed and promised to avenge the actions of Bhima. They attacked the Pandavas' camp later that night and killed all the Pandavas' remaining army including their children. Amongst the dead were Dhristadyumna and Shikhandi. Other than the Pandavas and Krishna, only Satyaki and Yuyutsu survived.