I currently own 23 games, I have beaten 12, 5 are sandbox games I pretty much just screw around in, the others are just games that I haven't gotten around to beating, but I've borrowed and rented a ton of games and I usually beat the, before sending them back, so I'd say I finish somewhere around 1/2 to 2/3 of the games I've ever played.
Usually the games I finish are the ones with the best stories,and RPGs like Metro 2033, Assassin's Creed, Bioshock, Mass Effect 1/2, and Dragon Age 1/2, IMO these games had stories I enjoyed amd wanted to see to the end, games that I really enjoy the gameplay, exploration, or just screwing around, like Just Cause 2, Fallout, or GTA I usually take my sweet ass time with and sometimes I just never get around to beating them.