Let's say you are trying to build or expand your professional network. This means staying in touch with everyone. If you send emails too often, your contact will likely be annoyed and unwilling to help you. If you never send an email, you will be forgotten -- getting in touch when you need something will be awkward and will seem opportunistic.
What is the sweet spot? Once a month, once a year?
I guess that would depend on who it is: how well you know each other, and how important/busy this person is (maybe even how talkative this person is).
Is it possible to find "categories" of people with a given range of "emailability"? (e.g. outgoing manager -> twice a year)
Or even better, some kind of formula? (e.g. months = importance_rate - outgoing_rate - familiarity_rate)