For me, it took about 8 hours, and that's because I spent time running around examining all the nooks and crannies of the incredible graphics in the map levels. I've read that it took less than 5 hours for some.
A common complaint was that the length of this game was too short, with which I concur, it should have been a bit longer. Maybe at least 3 or more average length levels should have been added to avoid the feeling that many got when they reached the end and that was: That's it?!?
So, how long did it take you in your first playthrough?