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2 factors for me really.
1) How long does it take me to get bored. I've quit lots of games just because I can't be bothered to even log in or do anything when I do.
2) How many of my friends are playing. If there's no one to do anything with I don't last near as long. If when I log in there's a bunch of people that want to do dungeons or whatever I can play for years if #1 doesn't kill the game for me :P
I tend not to be a solo player and have been in the same multigame guild for almost 10 years so a lot of what I play has to do with what friends are doing.