Expect to burn around 12 gallons per hour at full throttle. Back it down to 3/4 throttle for cruise, cut your consumption by maybe 40% to 45%. Assuming the engine is well-tuned.
How far you can go on those 12 gallons depends on your speed, which in turn depends on fuel quality, propeller choice and condition, hull design and condition, weight & balance, wind, waves and current, operator proficiency, and maybe one or two other things.
