I go by the meter on my transmitter.

One thing you can do is charge your batteries for 18 hours with the wall charger that came with your radio set. Then turn on your transmitter and receiver and find how much time it takes for your transmitter to go dead. That will give you a good idea of how long your batteries last.

I have a Futaba 4 channel radio and it stays on for 4 1/2 hours by doing that. On the other hand I have a newer version of the same radio and it only stays on for 3 hours. Same type of batteries in each but why one last longer than the other I have no idea.

Actual useage time can vary; depends on the numbers of servos you are using, how large the control surfaces are, how hard you are flying your plane, etc.

I have a falcon lll with just rudder and elevator, no ailerons and a 6 oz. tank and I have gotten up to 15 flights and it still had a charge in it on the older version futaba radio.

I started flying in 1973 and have never used a volt meter to check my batteries nor have I ever used anything other than my wall charger to charge my batteries...and I have NEVER had any batteries fail on me YET!

(If I fly 3 or 4 times in one day I give my radio a 7-10 hour charge if I plan to fly the next day. If I won't be flying for a week, I run my batteries down and give them another 18 hour charge and I am good to go for the next time out.)

You wanted an answer so that is mine!

Good Luck

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(I'm not an RC Pilot, I'm an RC Airplane Transmitter operator)