My boat has a functioning fuel
gauge. I also have a functioning hour meter. From 6 years of use I know that I use, on average, about 1/2 gallon an hour. This is for a Universal M25 turning a 3 blade prop, and the boat is an O'day 35. I typically run the engine at 80% of max rpm. On my boat that means I move at about 6 kts.
Considering that I use my engine about 50 hours a year I start the season with a fuel
tank, fill it again in August, and then at the end of the season.