If you feel like taking a day off, do so. If you feel like you're a little tired, but you still want to ride, just pay attention to your intensity. Take it easy some days so you can go harder on others. Sometimes I just don't feel like not riding. This is the end of a 70+ day streak last fall-- and I'm the first to admit I wasn't taking it easy as often as I should have been. But at least for me, ~20 miles with less than 1,000ft of vertical has become "recovery" for me.