Well, there's many possibilities
We've stayed at WL many times, and the boats seem to run approx every 15-20 minutes. There are both the larger boats that are enclosed, and the smaller brown boats that are completely open on the side. Not sure how it's determined which one runs, although my experience is that we've had the larger ones in the morning when it's busy and either one after that ----- I think we've had the larger boats most of the time.
The boats run in a route from MK to WL to FW and back to MK or the reverse of that. I've read that the route goes one way in the morning and the opposite way in the afternoon, but have never really paid that much attention. If the route is MK to FW to WL, it would take 15-20 minutes. If it's MK to WL, it would be approx 10 minutes.