Andy,

It isn't a straight line, because when you go to a larger battery you also add weight, but in an ideal world, if you were to double the mAh rating of your power source, lets say from 1500mAh to say 3000mAh, then you would double your duration. Unfortunately it doesn't work that way. It is a trade off of weight and longevity of the flight, verses the change in the planes performance. You also have to take into account the crick in the neck factor, when you double your flight time, and have to hold your head looking up for so long.

I try to plan my flight packs to give me 15 minutes of flight time at WOT. That a way, with a little throttle management, I can stretch it to an easy 20.

I hope that this helps.[8D]

Good Luck,
Darrell B.