If you want to be safe, I'd give it a good hour before you have to be there. It may take you much less if you don't have to wait long for transportation to arrive.
The best way to get there will be to take a MK bus from CBR. Once you get off the bus, you'll have the option of taking a boat from MK to GF, or taking the monorail. I've never taken the boat, so I have no idea how long it takes or how often they run. I can tell you that t he monorail won't be quick going from MK to GF. Once you get on the resort monorail outside MK, you'll make stops at Contemporary, TTC, and then Polynesian before coming back around to Grand Floridian.
Yup, a good hour oughta do it.
