Advertised battery life:
3rd Generation: Up to 24 hours
Mini: Up to 14 hours
Mini (Smart Cable): Up to 10 hours
2nd Generation: Up to 15 hours
To increase battery life:
Reduce screen timeout (Battery Saver)
Reduce screen brightness
Use downloaded content
Reduce volume
Turn off Bluetooth
Use playlists with long tracks
Reduce time before Auto Power-Off
Check battery level:
In Settings
By pressing the grey power button