I use a heart rate monitor for all athletic activity. I know that a low active HR after a tiring ride means I should stay active but cut the time and intensity back until my HR comes up normally with increased intensity. How much recovery time I need depends on how well I prepared for the bigger ride. When I'm training for an event ride, I keep it down to about 4 hours, but ridden as hard as I can and still finish. Doing the hard training ride once a week, I can ride double centuries, etc., no problem. It's more the intensity than the length of time that prepares one for distance cycling. IME. It's a little different than running in that the muscle damage from going hard is a lot less.
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Results matter