This is a tough question because I think it can really vary game to game. I think the two games I finished the quickest in this generation of consoles were Ico and Rygar. For some reason I was satisfied with Ico's length, but Rygar left me feeling a bit short-changed. I'm just about finished with God of War right now, and while it's taken me just a bit longer than those games, I'm completely satisfied with the pacing and length.
I've given up on games that pad out their length with too much backtracking, but then again it took me 5 months to finish San Andreas (mostly due to long stretches of very little free time), and I never really got bored with it. I was more than ready for something different by the end, but the game was still fun to me. I don't really play RPGs very often, so that long with one game is very unusual for me.
Even though most games I buy are $20 or less, I still like to feel I'm getting value for my dollar in the form of hours of enjoyment. I guess I'd have to vote "short games", with the idea that 10-20 hours is fine as long as they're 10-20 hours of fun.