Especially the older you get like I am who is now 65. I use to ride every day when I was racing, not even one rest day...well the one rest day was on the bike doing what was called easy pedaling for about 20 miles. As I got into my middle years I started to take one day off in the middle of the week, now I ride every other day, or sometimes I may ride more often but the "off" days are easy pedaling short days. I just base it on how I feel the next day, if the energy level is up it's a long hard day on the bike, fatigued then I either rest or do the easy thing. I know I can check my resting pulse upon waking but I've lived in my body for 65 years and I can tell what's going on.
There is more to cycling then just riding distance, a lot of fatigue can be controlled if you work in intervals into your riding schedule, and by doing intervals 2 or 3 times week (don't do these back to back) your longer rides will become faster and more productive.