Always. It's a curse, well an OCD. I usually don't choose what game to start, it's random or by other people, then I have to finish it and complete it 100% (or all the things I can possibly know if it's a very rare and obscure game). I can't play any other game from that platform, but I can play another game from another platform meanwhile if I get too crazy lol.
I don't consider achivements as 100%, even if they are native for the game, because I can or can't agree of what they request or consider what is a feat. My 100% is standard from old school, level up to the max, get all skills, all possible items, all sidequests, etc.
Also, if the game requires to be too skilled for going through I use cheats, because I don't like games that put you into a learning curve, that's a waste of time for me. I don't feel proud of beating something high difficulty, because I only count how much time I wasted doing something when I could have being doing something else, or playing another game.
With this approach, I have beaten 10-15 games a year (mostly PS1 games, because that's the generation I'm in right now).