I find that whatever electronic appliance I buy, the battery charge life is about 25% of that specified by the manufacturer. Trading standards should take a look at the advertising. I bought my wife a Roberts Digital radio for Christmas which cost about �80. A full set of batteries last only a few hours and cost �12 a time. She has to use it on mains which hardly makes it a portable as described.
I don't have an iPod, but I know with Apple laptops you have to run the battery through a calibration cycle (complete charge, discharge until it refuses to work any longer, and then recharge again) within the first week of owning the device. If it's the same with the iPod, and you didn't calibrate it, then you might need to get its battery changed.