A standard battery pack (500 MaH) should be good for 90-120 minutes on a full house .40 powered trainer.

I run a 600mah pack and an LA-40 in the LT-25 club trainer. I typically run a quart of fuel through it on a flight training day. This works out to about 8 10-15 minute flights.

With bigger engines, faster airplanes and more violent maneuvors, you wear down the batteries faster.