According to the tech specs
Music playback time: Up to 30 hours when fully charged
Video playback time: Up to 3.5 hours when fully charged
If you are doing a combination of things, 10 hours may be normal.
It's the screen that uses most of the power. If it is just playing music with the screen off, it uses relatively little power. If you are watching a video, it takes much more power. Audio volume level may also have an effect; higher volume uses more power.