as everyone's been saying, it all depends on the total weight of your boat. gear, how many people etc.
But to give you an idea,
- 24ft half cab boat about 2.5t (maybe a bit more if yours weighs about 2t)
- about 150L fuel
- about 100L water
- 3 people on board
- rev range between 3600 - 3800 rpm
Going from Moorebank straight out to trev alley and back, I used up under 100L. Its roughly about an hour each way with a few slow zones. So 15-40L per hour sound about right.