5 hours straight on Terraria when I first got it, similar thing with Minecraft in the early sessions. With more plot driven games, about 6 hours on Assassin's Creed II and about the same with Brotherhood.
I played Fallout New Vegas for about 8 hours one day, but I did stop in the middle to eat, so that doesn't really count.
CoD, Battlefield, Halo, and FPS can't really last more than a couple of hours, and only then when playing with friends