A full charge on the Hushh will last anywhere from 4 - 22 hours, depending upon which setting you’re using. Please note that the nightlight uses much more battery power than the sound, and that higher volume = faster battery drain.
A full charge on the Rohm will last anywhere from 10 - 22 hours. (This is because Rohm does not feature a nightlight.) Again, louder volume = more battery drain.
It is also possible to run these units while they are plugged in.