If you have a good charger that tells you how much you put back in you fly a couple of short flights and charge and that will give you an average consumption per minute, then you multiply the capacity of your pack by .8 and thats how many Mah (or a little less is better) that you want to put back in.

So if you have a 1000mah pack, you don't want to be putting back more than 800mah per charge or you're risking damaging most lipo batteries