Suspect you'll get a lot of opinions on this one. What I've seen is that many jet guys running only one Rx battery tend to charge after one or two flights just to be "safe".

I run 2 x 1650 mah or 2 x 1800 mah NiMH, lately configured in 5 cell packs, 2 switches, etc. I have run as many as 8 flights, 10 minutes "on" time each, ~8 airborne minutes, and still have around 6.25+ volts on the loaded voltmeter at the end of the day. I flew 3 flights on Monday, today is Thursday, and I just checked my readings and have 6.42v and 6.46v under load on the 1800 Nimh packs in the airplane.

You need a running history to gain confidence for capacity versus operating time. I use a max of maybe 125 - 150 mah per flight, usually less. That's the number you need to know. This is where a charger that tells you how much went in (or out) helps. Charge and/or discharge after flying to check how much is used. Use a loaded voltmeter to check health at the field or after flying. The higher capacity batteries also support the high instantaneous spike currents.

Another example: on my P-80, it uses 210-240 mah per flight depending on flight time and power settings. My P-70 uses about 160-180 mah per flight for about the same conditions. So on the 80, 4 flights on the 1250 pack is ok (I've done 5 occassionally) and on the 70, 5 flights is no problem.