If you just listen to music, it will last F-O-R-E-V-E-R.
Watching movies and using apps can drain it pretty quickly, as can having Wi-Fi ON.
If you want to keep the battery in good condition, make sure you only charge it when the battery is actually empty. If you always charge it when it is half empty or something like that, then you can actually wreck the battery, and you will have to get it replaced by Apple.
Kris