My longest sessions were always with a buncha friends, playing a game from start to finish. We did a 40-odd hours marathon on FFX from beginning to end, then killing off Penance (yes, we cheated. Or else, killing Penance would take closer to 200 hours continuous play). But that was with a friend though.
Another one was 10 hours Silent Hill run.
The longest was probably a 60 hours Lunar:Silver Star Story binge. Yes, we played the entire frikkin weekend.
My personal longest was definitely a LAN session with friends over CS, HL

eath Match and a buncha other mods. I think that one went on for about 14 hours. (We started bright and early in the morning, and went all the way through to late dinner)