Then I think you're getting quite a long playtime. The up to 18 hours battery life that Apple advertises is more like a maximum that you can only achieve by pressing the play button once and never skip forwards, backwards or navigate.
The iPod works in the way that it reads several songs into memory at once and then shuts down the hard drive to save battery. Whenever you skip a song or navigate to play from a different play list, the iPod will have to read songs again and the battery life will become shorter. Also, whenever the back-light is on, it will drain the battery faster, but I don't know how much faster.