The nano's battery should last close to 3 years. Lithium batteries have a life span of about 3 years. After that battery life will start to degrade.

Lithium batteries like the one used in the nano and all other mp3 players are capable of 500 full charges. One full charge means draining the battery completely then charging it to max capacity.

Now the nano has 14 hours of battery life. If you were to listen to the nano 14 hours a day every day it would only last about 15 months (1 year, 3 months). However it is unlikely that one would listen to an mp3 player for 14 hours a day everyday. So let's say you listened to it for 7 hour a day everyday. Then the battery would last 30 months (2 years 6 months). Now it is also very unlikely that an average person would use an mp3 player 7 hours everyday. I think a more appropriate number would be around 3 maybe 4 hours a day.

If a person where to use the nano 4 hours a day every day that would bring us into that 3 year period. All rechargable lithium batteries will start to fail around three years. That's just the battery's shelf life. Nothing can stop that.

So I believe it's safe to say that the battery in the Nano will last atleast 3 years.

Here's some valueble information

http://www.batteryuniversity.com/parttwo-34.htm
http://ipodbatteryfaq.com/
http://www.apple.com/batteries/

This information also holds true for every mp3 player that uses a rechargable lithium battery. Doesn't matter if the battery is user replaceable or not. All regarchable lithuim batteries function the same way.