On average a pilot can get 45-60 minutes of flight time for 3.5 litres of fuel. A standard tank of fuel at speeds up to 60kms an hour can get you 2.5-4hrs of flying. This, of course, can be dependent on the gear you are flying, overall weight, headwind, throttle-use, the size of the engine, fuel efficiency and glide ratio. Flying time can be extended for hours by cutting off the motor and using thermals, soaring and ridge lift to gain altitude. Switching back on as you need.