I would say that if you were to drink 16oz of water immediately before takeoff with that setup, your flight time would be limited, not by how long the fuel lasted, but how long you'd go before you had to land for a "pit stop"
Seriously, 16oz for a 20cc engine will go a long, long time. I have a 20cc engine (RCGF) in an old Extra 300, I fly at WOT much of the time. The fuel tank is only 10oz and my flight timer is set at 12 Min. and there's plenty left in the tank after that 12 min. flight.
Pete

OK, I see you're in Brazil. What percentage of Ethanol will you be running. I run the standard 87 octane 10% ethanol blend sold in my area