I've been doing some analysis on flight times with various configurations of motor, prop and wing. With our 15 kW motor, 125 cm prop, and 28 m2 wing, we are using 1.5 Ah per minute of flight. Generally then, to get an average of flight time per battery, and assuming that you only use 80% of the rated capacity, divide amp hours by 2. The best we have been able to achieve in winds of about 15 km/hr is about 1.2 Ah per minute. With our smaller 12 kW motor, the times are less by about 10%.