At our field, my instructor won't let you fly with a receiver voltage under 4.7 volts, for saftey sake (also, if your batteries go dead in the air, say good-by to your airplane, and just hope that it's headed away from people when it happens). And that's around 5 flights for my NexSTAR. Today, I was down to 4.7 volts after 4 flights, so I recharged at the field, and got two more flights, then went home for an overnight charge.

See Ya,
John