The boat ride over from Magic Kingdom takes about 10 - 20 minutes, depending on which stop is first - Wilderness Lodge or Ft. Wilderness. That is the boat ride only.
Your only variable would be how long you would have to wait for a boat. You could walk out to the boat dock and a boat is waiting there for you, and it leaves just as soon as you board....you could be at Wilderness Lodge in 10 minutes.
Or....you could walk up to the boat dock just as a boat was pulling away....Now, you may have to wait 10 - 15 minutes for the next boat to arrive, wait another 10 minutes for all other passengers to board, and then another 10 - 15 minutes for the ride over....