Depends what you mean by finished...
What if you made it to the end, but didn't collect everything?
What if there is more than one ending, but you only played one way?
What if you didn't do all the side quests?
What if you beat it on easy but not the hardest difficulty?
What if you played the single player, but never multi-player?
What if you played lots of multi-player, but never bothered with the single player campaign?
(51+ to 31+ depending on what you consider finished)