I know I must have had like a 24 hour+ gaming session at one point, and I'm pretty sure it was with Fallout 3.
I know what felt the longest though, when Left 4 Dead 2 came out. I hadn't slept all night before, and left the house at 7 am to go to work. Then followed 8 hours of constant standing up, working hard in a industrial kitchen. Got off, picked up my game, sat on the train for an hour to get home, ate and then went to bed. But I had to at least try one campaign right? I ended up staying up way into the next day, miserable because of the lack of sleep, running through the game several times. That was such a miserable night, but I just couldn't turn it off. Think my total "awake count" ended at 52 hours.