With my Mercury 115 2 stroke, my oil tank holds 1.4 gallon of oil, mixes oil at 50 to 1 at idle, 80 to 1 at wide open throttle, a full tank of oil will last 5 hours at wide open throttle, the low oil buzzer comes on when oil is down to 1 quart of oil, and that allows for 50 minutes of run time at wide open throttle.
According to the manual, and seems pretty close I recon...