Caches, clients, Facebooks, hosts, Metas, Redises(?), ... all those things don't really matter. What matters is B* reads from A, but how often should that be?

Caches, clients, Facebooks, hosts, Metas, Redises(?), ... all those things don't really matter. What matters is B* reads from A, but how often should that be? That's it, literally. That's the whole problem.