Using the buses, I'd allow an hour. It will probably be slightly less, but the timing of the buses is variable.
If you do not have a car and want to get there in a shorter amount of time, paid transportation is a faster option.
From MK, you have the option of taking the boat to WL. The bus is often faster, but not always.
Also important to know is that the WL bus also sometimes stops at Ft WL. I may have this backwards, but I think the loop order was WL, to Ft Wilderness, then MK. So if you are headed to MK from WL, you'll stop at Ft Wilderness before heading to MK. Coming from MK, the bus goes to WL first.
At popular transit times, the boat can get full, so you might have to wait until the next boat arrives.
The good news is that if you arrive early, they may well be able to seat you early, and WL is a nice place to hang out. If you run a few minutes late, they should also be a bit forgiving.