I usually don't go past 1:30 - 2 :00 hours, even if I have nothing better to do. I usually just get tired and end up not enjoying it as much.
If the game is in a really good moment I can stretch it to 3 hours. Rarely I go beyond this point, the exceptions are games which I find to be amazing or at least have a mechanic to keep me hooked and loose track of time.
Example: Final Fantasy X, Final Fantasy Tactics, SW: Knights of the Old Republic, Fallout 3, Dragon Age Origins, Mass Effect 2, Stella Deus, Valkyria Chronicles, Disgaea 1 & 2, Valkyrie Profile 2.
Noticeable is the amount of SRPGs, like I said, not all of those I think are amazing, but at least have a mechanic that kept me hooked for hours on end.