I've been out of the loop for quite some time as far as games go due to work, but I just discovered this site...may be old news to everyone else but I thought I would share.
Essentially it aggregates completion time of games based on user-submitted information. I just took a look at a few different games that I happen to remember how long they took me, and the average times on HLTB seemed pretty close. I know length of game isn't necessarily the most important thing, but I figured this would be a cool resource if you're going to rent a game or only have a certain amount of time and are trying to decide what to play.
Take a look and tell me what you think!