You should mention how many and what type of servos you are using and what the mAh of the battery is. For example, I have a typical .40 sized 4 channel model and it runs a battery down in about 4 - 15 minute flights (approximate). I also have a 1/4 scale Fly Baby with heavy duty servos. It wipes out a normal battery pack in 1-2 flights.

The battery voltage also drops under load. If you have a surface binding your battery will run out quicker and show a lower voltage.