The time really varies. They're supposed to last for around three or so years with heavy use (around 500 full recharge cycles, that is, dead battery -> full), but with light use, _should_ give 7+ years.
Two of my friends have had 3rd Gen Ipods since they came out, and haven't had any battery related problems, and I've had my 4th Gen Ipod for about 2 months now, it's great.
By the time your battery dies, you're probably going to want to invest in a new player that's more advanced and better anyway.