I'm getting about 8 11 minute flights on my 1650mAh 5 cell pack. This is with all digital servos. A 4 cell pack would last even longer. I'd also guess that you have a shorted cell in the pack.

I've got about 30 flights on mine now and it's holding up great. It even survived a deadstick from a hover at 10' AGL (barely).

I have about 3 degrees right thrust and the plane still turns right hard during harriers and when jamming on the throttle, so I've added another washer's worth of right for the next flight.