How Long Does It Take to Fix a Whole Photo Library?
Roughly what to expect by library size, and what actually makes it slow.
It depends far more on your disk than on the number of photos.
Reading a .json file and writing a date is quick. Copying the photo is not. A library of a few thousand photos finishes in minutes. Tens of thousands takes a while, and hundreds of thousands is an afternoon. Converting HEIC to JPG or re-encoding old videos adds real time, because those genuinely rebuild the file. Leave both off and the run is mostly limited by how fast your drive can copy. An external drive over USB will be slower than an internal one.
Test before you commit. Run it on one album, see how long a few hundred photos take, and multiply. There is a preview that shows what will happen without writing anything, which costs no time at all. The first 500 photos are free, and that is exactly the sample you want before starting a run on a big library.