Visualize how quickly the Tor network changes

I'm still not sure if this is a good metric for legacy/trac#2681 (moved), but Sathya asked me the idea in a metrics ticket some days ago:

Just brainstorming here, but I wonder if some kind of metric on how quickly the Tor network changes would help us decide if 3 days is a better interval than 5 days. By "how quickly the Tor network changes", I mean that if you take a consensus X from 3 days ago and a consensus Y from today, what's the percentage of routers in Y that are also in X (based on identity key)?

Such a metric could be a set of probability distributions that describe how likely it is for the Tor network to change by a specific amount in X days. So, for example, the probability distributions would tell us stuff like "Based on previous data, the Tor network has 40% chance to change by 20%, in five days." or "The Tor network has 80% chance to change by less than 5%, in one day." or "The Tor network has 40% chance to change by 35%, in two months".