Hey guys, long time lurker, first time poster. 'Sup.
Longest for me was just over 24 hours on Guitar Hero for a charity-raisin' world record. I thought the lack of sleep or repetitive strain injury would have been the worst bit, it was actually the motion sickness and bladder issues that nearly killed me. There's a vid of the highlights here: http://www.warlockmedia.com/gh-video.html
Thing is, I thought that was hardcore - I couldn't imagine some of the other comments (particularly SomeLameStuff's 70ish hour Mass Effect run). And they're not trying to prove anything either... just playing for fun, I rarely run past 8 hours.