I generally trade games in as soon as they are completed or am bored of them but during the last couple of console generation these are the approximate numbers:
Xbox360 / XBL arcade: 0 / 8
Gamecube 80+
DS: 60+
Wii: 18
PC: 2
I did used to keep all my games but now I only keep the ones I think I am likely to play again or have some reply value.
I still have the save files from the Gamecube and can always buy them again much cheaper than when I sold them.