Bhishma has a boon that When they came to their mind they could have left their life. I know that he did not leave his life immediately after He has many arrows in his body. When did he leave his body?
Bhishma lived for fifty eight days on bed of arrows. It was told by Bhishma in Mahabharata, Book 13: Anushasana Parva, SECTION CLXVII.
The mighty Bhishma then, taking the strong hand of Yudhishthira, addressed him in a voice deep as that of the clouds. That thorough master of words said, 'By good luck, O son of Kunti, thou hast come here with all thy counsellors, O Yudhishthira! The thousand-rayed maker of day, the holy Surya has begun his northward course. I have been lying on my bed here for eight and fifty nights. Stretched on these sharp-pointed arrows I have felt this period to be as long as if it was a century. O Yudhishthira, the lunar month of Magha has come. This is, again, the lighted fortnight and a fourth part of it ought by this (according to my calculations) be over.'