5 days
IIRC (I have used this 1 time in a decade) this normally does a couple of books per second
Restore starts with building a fresh
metadata.db by using
metadata_db_prefs_backup.json to recreate your customization/configuration.
Then it recursively reads
metadata.opf from each author\book folder it finds. (that is what you see normally in the grid view)
I would start with a (windows) DISK CHECK and make sure the filesystem is intact. That is all I can think about it going off the deep end.