It is usually 3 hours, but they want you ready about 15 minutes before that time. So if your scheduled pickup time is 3:00 pm, you would need to be at the stop and ready to board at 2:45 pm.
I would suggest leaving MK for WL about 1:30. That would give you time in case you have any unusually long waits for a boat or bus to WL or getting any stored luggage. I find that midday there seem to be less boats running so there might be a longer wait. Plus, I once had to wait about 20 minutes to pick up some stored luggage at a resort because of others in line.