23 hours of playing Paper Mario from Start to finish. I started at 10 in the morning, and ended at 9 in the morning the next day. I felt so awesome after that. I needed Gatorade to keep me awake.
My longest gaming session had to have been on World of Warcraft and I played for almost 24 hours and was grinding reputation the entire time. But I usually have really long gaming sessions that last anywhere from 12-16 hours whenever I'm really into something.