Is there anything wrong with just doing a timed run on the ground? Meaning, secure the plane on the ground with a rope to a pole, you just want to hold it in place. Run the plane at half throttle while you time it. The ESC should shut the motor down when the voltage gets to 3.5 (if it is a good ESC, check your instructions). Then do another run at full throttle. This should give you an average time.

I would do it in the cool part of the day if possible since I am not sure of heat will be an issue. I'm a noob to electrics also.