I like 3 hours, drifting to 3.5.
However some of my friends fade out at 2.5
Online it seems to need to shorter, so 2.5 for me, 2 for others.
But.. at a con I can play 14 hours in 3 sessions with an hour in-between.
I just need power naps every three hours really, but I am in my mid fifties so...