Non-stop gaming only, so getting up to take a ten minute break doesn't count. If you had to stop to use the bathroom, or get a drink/snack, then fair enough, but stopping for half an hour to cook or eat a meal doesn't count as a single session.
Personally, the best I've ever managed is about 9 hours, starting at 9am in the morning and ending at 6pm when I finally had something to eat. I'd not long had Tales of Vesperia, and the story was starting to get interesting - my brother and I couldn't tear ourselves away from it.
Sadly, I haven't had long periods of time of late to play games, so I haven't pulled off any marathons of that magnitude for a while.