It will probably take a long time.
Noone knows exactly how long. It mostly depends on the dev.
Facebook for example, takes the longest to update and I use Messenger and Instagram all the time, so that's a bummer. Same goes for Tapatalk that is my 3rd most used app.
I remember that it took many months for some apps to update for the XS Max resolution and I recall Apple having put a maximum time limit for developers (March, I think). While I haven't read about a time limit this time around.
I have yet to read a single article from the big sites, concerning 3rd party outdated app resolutions of this year's devices. I even remember there was a pinned thread on MacRumors, where people posted everytime a new app got updated.
I don't notice users being bothered by this, this year. There was a lot of whining back to XS Max era. Maybe it's not so noticeable, due to the resolutions not being drastically different? Maybe people are still blinded by the beauty of their device, not noticing such details yet?
This was one of the reasons I canceled my already paid order, aside from other bad things I've read in the forums about the new devices (green blacks being the most serious).