M_Boats: outboard motor fuel usage

Hey out there in Monty land' I have a M-17 with a 4 hp, 4 stroke, Nissan outboard. It has an internal fuel tank that holds about 1/3 of a gallon. While I don't do a lot of motoring, I have been trying to figure roughly how long I can run on a tank of gas. I don't have a manual and I can't find anything on Nissan"s web page. The tank is poly and is really odd shaped so tank calibration is not a possibility additionally, to sight the tank I must shut down and remove the remove the cover. Short of doing a test in which I run around in circles until I run out of gas, does anyone out there have the same motor and some similar experience. Thanks..Arnold on M-17 "LITTLE BREEZE"