From :
http://www.fiio.net/en/products/89/parameters
They state it in their specs, and include the audio file they used as well:
Battery Life & Test Conditions
3.5mm headphone output:≥20 h
Volume 15 Load M3 open earbuds (27 Ω)
EQ OFF Screen OFF
Test File MP3 44.1kHz/16bit
Bluetooth output:>26 h
Volume 60 Load 32Ω
Test File MP3 44.1kHz/16bit Screen OFF
Battery life test file download link
click here to download
If you take real world use with screen on and off all day, then it will come up a lot shorter.