Aside from a game or two or Tetris before I go to bed, I usually refrain from playing videogames until a large block of free time comes along, allowing me to achieve complete immersion in the game for 6 - 7 hours straight. If I'm really, really into the game, I'll usually break my own rules, and let it creep into my normal weekday schedule, but otherwise, gaming occurs on the weekends and during chunks of free time that occasionally pop up.
I'm a bit of a perfectionist who can get overly competitive, so I'm never gonna tough an MMORPG if I can help it, because I know it'll devour my soul and social life in one quick gulp. Games requiring a lot of strategy often interrupt my normal dreams as I sleep, so too much gaming, MMORPG or otherwise, isn't a good thing.