We stayed at WL earlier this summer. I would recommend about 30 minutes from MK to WL. You need to allow time to walk from MK to the dock and from the WL dock to Whispering canyons. It is only a few minutes on each side but added together, it is 5-10 minutes. In addition, you need to budget time for waiting for the boat. Sometimes it is zero minutes but it could be 10-15 minutes. The actual boat ride is about 10 minutes.
Walk time to/from docks - 5-10 minutes
Wait time for next boat - 10-15 minutes
Boat ride - 10 minutes
Range: 15-35 minutes
So, budgeting 30 minutes is the "safest". It may be faster but you can't be sure.