It's impossible to say exactly. The reason is the type of flying you're doing, lazy circles in the sky will use much less power than aerobatics, the size of the plane, and the number of servos will all determine the amount of battery used. I bring a battery tester to the field and check the voltage BEFORE the first flight and then regularly throughout the day. When the voltage gets to a certain point I stop flying the plane. Testing the voltage is easier if you install the switch and charging jack in something like this: http://www2.towerhobbies.com/cgi-bin...&I=LXJ778&P=ML