The resort launch takes 5 minutes from leaving the dock at MK to arriving at the GF dock. There are always two resort launches in service and the time in between boats is about 10 minutes. So, the longest travel time is about 15 minutes. I've timed this many, many times over many trips and it's always roughly the same regardless of time of day. Just add about 5 minutes for the boat to load at MK and another 10 to get to your restaurant once you arrive at GF and you'll be fine.
The absolute worst way to go is the monorail which has stops at CR, TTC, and the Poly before GF. It's very much longer.