It depends.
I pretty much look at the game and think "Will I ever want to play this again?".
If I like a series in general, I'll buy and keep all the games regardless of if I liked them or if I will ever play them again. It's literally the only reason I still own the 3 Final Fantasy XIII games. I would have sold them otherwise.
If I don't think I'll ever play a game again or just flat out didn't enjoy it (and it isn't a sequel to something I like) I'll sell it straight away. This means any primarily online multiplayer based games get sold as soon as they become dated.
This happened way too much with the last gen consoles (PS3/360)....The amount of games I've kept on PS3 compared to PS2 is pathetic.