I like to keep them at least a year. That's how long it takes to really get to know a game. If it's less than that, I hated the game. If it's longer than that, I really like the game.
Right now there are quite a few games jamming up the works. Games that I haven't moved because I've been busy, or games I'm trying to hit some last objectives on before moving. But I'll probably sell 3 or 4 games in the next month or two to clear up some space (that I will promptly fill with new ones).