I agree with 90 minutes at the upper limit.  Since it is a glider, you should have very low current draw.  Best way is to go for the 40 minutes, then test how much is left on the pack by discharging into a 20 ohm resistor.  When the voltage drops to 4.6 volts, stop the test.  It should go 40 minutes in the air and at least another 40 in the test.