As with the likes of other special villagers such as Label and Saharah, Redd will also appear on your island at random. He randomly appears on your island and there is no specific time frame for when he visits. However, you can expect to find him wandering on your island once every 2 weeks or so.

How often does Redd appear in ACNH?

Like most of the vendors in the game, he has a habit of appearing quite randomly throughout the weeks. Some players on the Animal Crossing subreddit have reported that he has appeared every few days for them, while others have taken up to 19 days (time traveling) to make him appear.

Does Redd show up randomly?

As with the likes of Saharah and Label, Redd will appear on your island at random. There are two ways to know he has appeared. One, Isabelle can announce his presence as part of the morning update as a "suspicious character", but Redd can also just appear without any kind of warning.