I’m in the US, as are most of these companies, so that’s generally what’s being discussed here. I don’t have any experience with other countries’ copyright law.
But for the US, it’s intentional that there isn’t an exact objective threshold. The fair use tests are subjective, to allow use of a copyrighted work in artistic and other non-commercial uses. And, as you mentioned, incidental inclusions in personal photos.
I’m in the US, as are most of these companies, so that’s generally what’s being discussed here. I don’t have any experience with other countries’ copyright law.
But for the US, it’s intentional that there isn’t an exact objective threshold. The fair use tests are subjective, to allow use of a copyrighted work in artistic and other non-commercial uses. And, as you mentioned, incidental inclusions in personal photos.