I can't answer for certain, but the Statue of Liberty is about 6 miles from the passenger docks as you exit the bay. The trip, will take you past most of Manhattan. I would imagine that the ship will be going really slow, and so my educated guess would be about 1 hour, because the ship will have to back out, and get into position to move on its own. It is a crowed area and it would be only moving about 10 mph at most I would think.
If you have the time, I would really recommend you going to the Statue, and Ellis Island as a side trip.