Arx Fatalis might be a good choice for you then; it's really short as far as RPGs go, especially if you aren't the sort to poke into every corner. Even completing all of the handful of side quests won't add that much to the play time.
EDIT: note that when I say it's very short I'm not saying that's a bad thing at all; I enjoyed every minute of it, with my only real complaint being that I wished it was longer. I like my RPGs fat and time-consuming. ;)

I own around 30 games from GOG and so far I've finished 2 :D
So I would say you're quite efficient... ;)