It completely depends on the game. And I'm only focusing on PC gaming here, because: yeah.
Let's take strategy on the table:
Rise Of Nations is my favorite real-time strategy, ever. You dive very intensely to it once you get the general hang of it. And there are so many things to focus on and to remember that it has offered some of the best gaming, and flow experiences I've had. But as it is intense; after a one game, for about 40-60 minutes, you emerge back from the game and with the intensity, you'll want to have a break at least before having another go.
Civilization (IV or V, in this case) is the unquestionable king of turn-based combat. With the much more relaxed pace and endless & intriguing contemplating of matters regarding your civilization, it's relationship, economy, warfare, culture, the list goes on and on.... With all that, accompanied by the healthier pace, you can play it for many hours indeed and notice your thinking one-more-turn, and it's the morning already.
FPS's can also quite tolling, as they are generally active games. Even there are differences. Games where you don't instantly respawn, so you'll automatically have breaks, but might have to think more, like Counter-Strike: Source, can be played for more than 3 hours if you've got the right mindset. But usually doses of about one hour, are optimal. With instant respawning, in games like Battlefield: Bad Company 2, you might notice sometimes having played more than you would play, because the action is more or less, constant, hell, even the loading screens for the next map, offer you interesting statistics, so the attention doesn't brake.
I also smoke, so I'll have 5-minute breaks every hour or so, making games less straining and continuous. But I do agree that many games, especially RPG's or anything campaign-driven, can turn into a chore, after a time. The similarities and the same type of situations, quests and so on, reoccurring to a much more noticeable degree while gaming for longer and being much more familiar and in touch with the game, seem to pop up much more often and it really can become a chore.
So, I usually play from 1 hour to about 8 hours, if I'm really into it, and as being a kind of overkill person at times. And games most certainly can become a chore (I don't see MMO's as anything else than an annoying chore) but it depends heavily on the game, situation, environment and the person him/herself.
