I bought Deus Ex very cheap, at what is the Brazilian equivalent of a magic dollar store, figuring that it was worth it even if my computer couldn't run it, because eventually I ought to own a computer capable of playing it. My computer fried and I had to get a new, better one at the end of that year. So, I'd say nine, ten months.
Also I think I had Yahtzee's Days trilogy for maybe two years on my computer before I actually played it. Although I have several free games in a similar situation, so it may not count.
I also currently have several games I have bought and downloaded and not played, because due to time constraints I only play my indie/classic PC games when I don't have a 360 game to play. The list currently includes Fallout, Fallout 2 and Myst.