The recent update to Animal Crossing New Horizons added Redd as an occasional NPC. According to this guide, the first time he appears, Isabelle announces it.
The problem is, I forgot to talk to him on that day, and he hasn’t come back since.
How long do I have to wait to get him to reappear?
EDIT: To be clear – my understanding is that to have him appear in his boat, you need to buy the initial artwork to open the art gallery in the museum. I didn't do that. So now Blathers keeps saying "I'd love you to donate art" but I don't have any art to donate!
Best Answer
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