Ok, so I never really timed it so I could be wrong but I would agree that 15 min seems about right. It was long enough to actually relax and enjoy the ride but not so long that you wondered if you would ever get there.
10 - 15 minutes is about right. But, if you arrive at the dock and see the boat pulling away, add about 15 - 20 minutes waiting for the next boat before you arrive at the Magic Kingdom dock.