Even an iPod should not need a new battery as often as every 18 months. As I mentioned in another thread, a LiIon battery should give about three years service if charged every night. The main reason the batteries don't last as long as they should is due to improper charging or discharging a battery too far.
Here is my rule of thumb. If the device is used daily, then put it on the charger at night. If it isn't used that often, then put it on the charger when the battery level gets to between 1/4 - 1/3. Don't leave it plugged into the charger for days/weeks at a time. Unless I have a defective battery, I always get maximum life this way.