Once again I've got to agree with Piper_chuck. That does seem a bit short for battery life. Let's not worry about the battery for a minute. Things that can affect the life of battery can actually be mechanical problems with the plane. A binding of a control surface can cause the servos to have to draw more power to push/pull that control surface. A servo that is going bad can draw more power than normal and drain your batteries. While not very common these things can cause problems with your battery and shorten your flying time. More than likely you have a bad battery. It's possible that this radio sat on the shelf for awhile before you got it and the battery is old. The best thing you can do is see if you can find somebody that has a battery cycler. This will drain the battery and measure how much time it takes to drain. This will represent how much flying time you will have out of that battery. If it matches the time you are getting in the air then your problems are with that battery pack and it's time to get a new battery. When you buy a new battery one with a higher capacity and I would recommend getting a NiMH pack instead of a Ni-Cad battery. I have 1600 MaH NiMH batteries in my 40-60 size planes and I can usually get 50-75 minutes of flying time out of them each time out to the field.

Hope this helps

Ken