The voltage cutoff, if there is one, will probably be down around 5 Volts, which is the minimum necessary to power the receiver and servo(s). All this means is that you will have to be careful. Fly for a few minutes, land, measure the voltage. Keep track of how long you fly. Do this until the voltage is down near 6V at rest. Add up total flying time to get a good idea of how long you can fly.