Time (hours) = Battery Capacity (mAh) / Current (mA)

Standard 9V battery capacity: 565mAh

Average music listening volume current draw: 25mAh

22.6 Hours = 565mAh/24mA

If using the Crush Mini as an office speaker, it is worth powering with wire and a not battery since it would require a new battery every 22.6 hours.

However, most inexpensive switching-type power supplies are noisy, and that noise is audible through the amplifier. A regulated linear power supply would be ideal. I was able to use a benchtop switching power supply with a lower noise floor. It is tolerable but not a permanent solution.