The Kurukshetra War lasted 18 days according to the Mahabharata.
The war was fought between the Pandavas and the Kauravas over the Kuru throne and is the central military conflict of the epic. It also provides the setting for the Bhagavad Gita, in which Krishna counsels the troubled warrior Arjuna before the fighting begins.
The battle’s command changed as major leaders fell. Bhishma commanded the Kaurava army for the first 10 days, Drona for days 11 through 15, Karna for days 16 and 17, and Shalya on the final day. The Pandava commander-in-chief was Dhrishtadyumna.
On day 18, Yudhishthira killed Shalya, Sahadeva killed Shakuni, and Bhima defeated Duryodhana in a mace duel. The fighting did not end all violence immediately: later that night, Ashwatthama, Kripa and Kritavarma attacked the Pandava camp. Thus, 18 days refers to the main war, not every subsequent act of vengeance.