I think it's similar to adding a note to a book, and then removing that note, leaving you with a the original book. That scenario doesn't violate copyright law as far as I'm aware.
So around 30 years now.
Playing the title you backed up and patched is a different matter entirely
You could design a system that generates undo info while you're patching. But it's easier to make a full backup copy of the original rom, and have no way to actually reverse the patching process.