Infamous took me about 20 hours on my first play through, it's also really repayable and that's why I finished the campaign another two times after my first. Infamous 2 just came out and I've heard it ain't to bad.
Uncharted 2 took me about 9 to 10 hours, played through it twice, it's not as repayable in terms of new things to do, but it is an extremely fun game to play and it's worth a second run.
Gears of War 1 and 2 went on for about 10 hours a piece, both have co-op and they're fun.
Mass Effect 2 was about 20 hours, it offered a unique third person experience, which basically means that it strives for interaction and storytelling over third person game-play, but it was still fun and it lasted a while.
Red Faction Guerrilla is a fun game, the campaign can get repetitive and boring, but the missions in between are really fun and it's a game that strives on game-play and environments. Red Faction Armageddon is a turd of a game, played it for an hour and returned it, just horrible and I don't recommend it.
Dead Space 1 and 2 are about 10 hours each and are both amazing games. I highly recommend them if your looking for really atmospheric and fun third person campaigns, it also probably helps that you can pick up the first for about $10 now.
Vanquish is the best playing third person shooter on the market, in terms of enemy AI, level design, game-play and visual effects, it trumps every other shooter I've listed. It's only about 5 hours long though, but the replay value is extremely high, it's structured like a SHMUP, which means it warrants multiple play throughs just so you can beat your times and get higher on the leader boards. Just as an example, I played the challenge demo about 40 times before I bought the game, it was that fun and amazing.
Resident Evil 5 is really good, it plays slightly differently to the other shooters I've mentioned, but it's more fun because of it. Don't listen to what others might tell you, it's the best in the series and deserves a shot.