If you're playing Little Big Planet 2 just as a game, you're missing out on the big picture. While the story mode itself is roughly about 7-8 hrs of gameplay, the levels themselves are so creative that you'll want to play them over and over again in awe. Just think...all the levels that were designed for the game were created with the very same tools at the players' disposal. In a very defiant way, Media Molecule just demonstrated their upgraded toolset with this sequel.
Earning more objects and stickers to use in your own imaginative levels are another incentive to replay the story levels again, especially when some of them require more than one player to achieve.
And even if you're the more reclusive type, there's plenty of levels online by other players that will last you quite some time.