Standard advice is 24 hours, but most likely you are in the same situation as ths guy.
https://www.mobileread.com/forums/sh...d.php?t=287034
He flashed paperwhite3 5.7.4 to a PW3-32GB which was not supported.
It looks like he fixed his problem by buying a new kindle.
The update packages all contain a 'uBoot' boot loader that knows how to talk to the memory and storage on your device, and if you install the wrong one, you get this kind of thing.