Normally, I get about 3/4 of the way through games, and decide that I've gotten as much fun as I can out of it. It's at that point that I:
a)Return the game to Blockbuster,
b)Move onto online play, or
c)Let it collect dust on my shelf.
Unless I am truly driven to find out the ending of the game, or if I REALLY want some award, (COD 5 Nazi Zombies, for example,) I'll rarely finish a game.