I would definitely pick up Killzone:Liberation off of the Playstation Store. There's a free update that you can get through the game that adds the option to play against people via Wifi, and there's always around 100 people playing. The patch also includes an extra level, all of which makes it a killer purchase. You will easily get 8-12 hours given the difficulty (Guerilla Games are wicked AI programmers)
Not to mention a great looking game for the platform.