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

I have had Redd appear the second next day after he initially appeared on my island. You can always check if he is there by looking at your map since his trawler does already appear, even if he did not open it for you already. His trawler has a shop symbol located at your islands secret beach.

I'd guess Redd will be like C.J. and Flick. Where one of them appears every 3-5 days on your island.

But maybe he even got his own unique spawn conditions and you can have him, C.J. or Flick and one of the vendors appearing at the town square at the same time.