I don't feel the need to be too systematic about it. Typically, I'll average about one day off each week, but sometime I'll ride eight or nine days at a stretch, sometimes only four, depending on which places look interesting and worth taking some time to explore.

I recently did an eight-week tour. Of the 56 days of the tour I was on the bike for 46. On two occasions I took two successive days off, so effectively I had eight rest stops, two of two days each and six of one day each.

As to substituting easy days for rest days, I don't usually do this simply because I make a point of stopping in places it is worth stopping in. Easy days work fine, though. Personally I always feel a bit sluggish the day after a rest day, as if it takes a few hours for my muscles to get back into it once they've relaxed. Interestingly, the pros in stage races experience the same thing. On their "rest days" they all spend at least a couple of hours on the bike, saying that if they don't do that they feel heavy legged when they restart. But when I'm touring I am not in a hurry, so this isn't really an issue.