There is no real formula, just some common sense based on your flying style.

If you are heavy on the sticks, meaning if you do a lot of corrections while flying, your servos will work more and you will eat up more battery power. Conversely, if you are easy on the sticks and allow the plane to "fly itself" in straight lines, meaning no over-correcting when no corrections are really needed, then you will get more flight time out of your batteries. You would be surprised at how often RC'ers try to correct something that is perfectly normal

But, I have several planes with 5 servos and use a 1200mah NiMH 5 cell pack and get four or five flights before I put it on the charger. By that time, I've either changed to another plane or am ready to head home.

We sit around and shoot the bull between flights, so the time goes by, and four or five flights is plenty for one day.

I've always recommended the use of the Voltwatch on your planes. It gives you a quick look at the battery condition before and after each flight so you can decide if the last flight was the last before you recharge or if you are ok to fly again.

http://www3.towerhobbies.com/cgi-bin...&I=LXHDJ2&P=ML

CGr.