On a typical trainer, with typical 600mah pack, and typical trainer servos flown in a typical trainer way, I agree with what others have said. 4 10min flights, or 3 15min flights.

Larger batteries are nice, but remember that you have to charge them longer or on a faster rate charger, your typical "wall wort" isn't really up to it anymore.

I use a fast charger at the field all the time. Last sunday I put around 10 flights on my trainer, recharged the flight pack twice .