I don't think there's going to be a consistent definition. Part of the idea of what is a ride or what is a century involves the course, the purpose, and other factors besides just riding time/ resting time.

For example- suppose you ride from your home town to some other town 100 miles away. As long as you did that in the same day, most people would count it as "one ride", regardless of how many stops you had, or how long those stops were for. On the other hand, if I ride my bike to Walmart and back, wait 5 minutes, then ride to the post office and back, I'd count that as two rides, not one, even though I was only off the bike for 5 minutes. There, it's not the time, it's the fact that the two rides are for different purposes.

It took me about 9-1/2 hours to do the Hotter-n-Hell 100 last summer. Some people can do that in about 4 hours. If one of those guys rode for two hours, took a 5-1/2 hour nap, then rode for two more hours and finished the course, most people would call that a century, since they completed a recognized course. But if you ride a 50 mile loop in the morning, sit around the house for 5-1/2 hours, then go ride a different 50 mile loop in the evening, that's two rides, as they have separate routes, etc. The degree of difficulty may be exactly the same either way.

If the point of tracking the centuries is to keep track of your workouts, your effort, etc., it doesn't really matter how those miles were arranged or what the purpose of riding them was, though, and for that purpose, making 50 two-mile trips to Walmart ought to count just as well as a single long ride, even though you might not normally call the 50 two-mile trips a century.