I missed Redd on his first visit to my Animal Crossing: New Horizons island. When does he come back?

As with most visiting NPCs he'll likely show up at random. I don't know how much testing they have done, but Gamewith seems to think around every 2 weeks or so:

https://gamewith.net/animal-crossing-new-horizons/article/show/18085

He did eventually return; he still appears on the plaza until you buy from him for the first time / the museum upgrades (not sure what would happen if you buy your first artwork from a friend’s island), but Isabelle doesn’t announce it. I didn’t record when his initial visit was, but the return was probably around the two week result that others have mentioned.

You seem to only be able to have one of the extra NPCs at a time. So if you are being visited by Flick, CJ, Leif, Sahara, Label, Gulliver, Wisp, etc that day, I don’t think he appears. That may well be why it takes so long to reappear.