We already answered that McCloud.
Also, I forgot to mention a variable:
Trophies, Stickers, CDs. If you are going to go through it while getting them my estimate is 9, 10 hours, somewhere around there. If you are going to go through adventure mode without bothering to get the extras, I would say 6-8 hours.
Of course, the time can change drastically if you die alot, or not die at all.