I don’t intentionally plan rest days. In my experience I have found the best way to recover from a hard ride is more riding. I’ve had days when I have been almost too sore to lift a leg over my bike, but after a few miles all is good.
When planning a long ride, I do add a rest day or days, depending on the length of the ride, and where I am riding. If the weather is simply too bad to get on the bike (rain, wind, falling trees), or I get a case of Montezuma’s revenge and can’t be far from a toilet, those days come in handy. If the weather is good, and I don’t get sick, I can do some sightseeing, or spend time with new friends.