Take crashem's advice and do a real life check. Depending on prop, needle setting, etc run times will vary to anyone else's experience.

I have a hand crank pump and here's what I would do. Fill the tank, counting the cranks in. Start the engine and run wide open for 3 minutes. Stop the engine and refill tank counting number of cranks to fill it back up. Then just do the math.

10 cranks to fill
3 minutes of run wide open
5 cranks to refill
equals
6 minutes at wide open.

Now go ahead and fly with a 5 minute timer as you would normally fly. Refill it and count cranks again. Revise your flight times based on your actual fuel draw with normal typical flying style.

Duke