I haven't done many super long gaming sessions as I am weak and don't function well at all when sleep deprived. However I did do about 17 hours or so once when Wrath of the Lich King launched on WoW. Not one of my finer moments, but since we're admitting things!
My longest gaming session with one game would only be about 10 hours that I can recall, and that was playing Twilight Princess when I first got my Wii. I know for a fact that I have spent longer times playing other games with my nephew in the past, but we never really time those to see how long they last.... Lately I don't think I could go that long, though, as I've been getting ill a lot recently and so I've been taking really long naps all the time, and wouldn't be able to stay up that many hours.