The question may be simple, but the answer is a little bit more complicated depending on flying style, friction in the control system, size of control surfaces, etcetera.

In most of my .40-sized sport aircraft I use JR 4.8 V, 600 mAh NiCd batteries.
As an old time C/L-flier I am perhaps more careful than most when it come to reducing friction in the control system. Often I use nylon coated wire in a pull-pull set-up for rudder, balsa/metal pushrods for elevator and ailerons and wire in plastic tube for throttle.
My style of flying can be described as "traditional pattern aerobatics".

I have found that at most I can get 10 fifteen minutes flight out of a freshly charged Rx battery pack. Usually I call it a day after eight 15 minutes flights.

If you have access to a discharger that can tell you the mAh it is a good idea to perform a couple of tests. If you consider 4 flights safe, with a freshly charged battery perform two normal flights. Discharge the pack at C rate (0.6 A for a 600 mAh battery) to 1.1 V/cell. Charge the battery and do four flights. Again discharge the battery pack to 1.1 V/cell and compare the capacities remaining. If after two flights 400 mAh remains and after if after four flights 200 mAh remains, it is obvioius that two flights have used up 200 mAh, i.e., 100 mAh per flight. Thus a 600 mAh can be used for at most 6 flights. Add a safety margin and I would say that 5 flights should be O.K.
Because batteries age, one should repeat the test now and then, say every six months.

Most battery checkers are very conservative when indicating "battery low".

Edit:
Make sure you use a battery checker that actually puts a load on the battery. Some battery checkers don't and require you to "wiggle the sticks". I use a simple digital voltmeter with a 6V, 2.4W bicycle light bulb as a load when measuring the voltage. I stop flying when the voltage reading falls below 4.4 V within 15 seconds of attaching the voltmeter.

Disclaimer: The values indicated above works for me, but it is up to you to decide how many flights you consider safe with your battery, aircraft and flying style.