Nimbus itself does not have a charge or built-in battery life. It is an AC-powered smart amplifier.
If you use a third-party portable power station, the runtime depends entirely on the capacity of the battery pack you choose. For example, a 150Wh pack may power Nimbus for a couple of hours, while larger capacity packs (300Wh and above) could last much longer.
Since this depends on the external pack, charging time and total runtime vary by the device you purchase.