I think the question of length is directly tied to cost. If you're dropping $60, how much are you getting in return? A lot of FPS games can be blown through in 8-10 hours and that seems a bit short for $60 IMO. A movie costs about $8-10 for 90-120 minutes or so. By comparison, if you wanted 10 hours of entertainment from a theater you'd be spending about $50 for 10 hours or so. I suppose that's on par. I tend to factor in the multiplayer too though. I dropped over 13 days of online time with COD4 so I think I got my money's worth.
Now, do some games drag on for far too long? Maybe. I'm having a hard time remembering the last game that I wish was shorter. Generally people don't say "Wow, that was a great game if only it had been shorter!".
Also, developers need to stop shamelessly padding their games to make them longer. Scavenger hunts for 350 pieces of intel doesn't constitute another 10 hours of gameplay to put on your box so you can say "Over 100 hours of gameplay!". Most of the time I can overlook it but some games (Zelda Wind Waker, Darkstalkers) artificially pad out their game with back tracking and such because they clearly didn't have time to finish the game.