I've got a 3rd generation iPod, I got it in 2003, so my experience may be different from yours since my iPod is older and different.
But I've found a lot of things affect the battery life, like if you leave it in your car in the cold it will lose battery power faster, or if the backlight is on, it's on repeat and shuffle then it drains it faster as well. I bought a car charger that plugs into the cigarette lighter so I never charge mine at home much, I just charge it in the car, which is probably bad because it's not a continuous charge like southerndoc said.