Almost all files suddenly got deleted, file in storage.list() but doesn't exist when I try to read it

How long would this typically take, and is there a message during boot when this happens?

The check happens reasonably fast at boot time, but if an error is found it'll display a message and erasing and writing the JS could take a minute or so.

It's possible it could have been interrupted but I don't think that could cause the corruption (worst case it'd have failed to write some files that should be there).

And what would it be resetting to?

It's exactly as it comes (with that firmware version) so welcome screen, and very basic apps.

Glad it's sorted now at least - but sorry you hit those corruption issues :(