I am thinking about doing the 100% category, but when should timing stop? There are no credits after finishing all stories. The credits only play after your first story completes.

You need to replay one story after them all to get the last dialog that says "this day you reach the end of a story." That makes 13 total playthroughs.

Here is the end of the 13th playthrough for reference:

I was thinking maybe end time when you click to enter the house after 13th playthrough? Or if you don't think this final dialogue matters (the platinum actually pops up at the end of 12th playthrough), then maybe when you click to enter house after 12th playthrough and make sure video includes the scene where it shows all the books and says the shelf is full just for verification? Would hate to have to set up a new profile for every run if it went by when the platinum trophy pops up on screen (that just makes the category even less likely to be played and I seem to be the first even considering it), but that is also an option.

It's worth noting there is a bug at the Leviathan scene where the game can randomly softlock. It happened to me while I was getting all trophies. I think my routing would start with the two Leviathan paths first because of this. I also found a softlock at right-side cave exit.