I have rented the pc 474 about 6 times for 7 - 9 days and run all over BVIS - including anegada. use generator every night at least. so The most fuel was 120 gallons. least 80 gallons. they have 150 Cummins, cats or JD and I cruise at 2000 - 2200 = 10 knots. They are very fuel efficient and the genset is about a quart and hour or so. I also ran a 393 PC for 6 days about same and it was 60 gallons.
If you get a 51 PC or especially the Marine max 484 and run it hard it will suck 200-300 gallons the fuel dock guys tell me.

Key is not be in to big hurry and you will enjoy the ride and use the auto pilot.

Hope this helps...feel free to PM me if you want. BTW I just bought a pc 433 with Yanmars 260 in it I am going to be using over thanksgiving. I have the fuel burn rate charts on it already but will see in real life.