I don't know any standard of what's too much/little, as it depends on the relationships between the source and audience. Marketing can be a bit draining, so fewer is the rule. But if it's a coach and client on a specific goal, you can get away with quite a lot of communication. Also, if people are being held accountable, and meeting someone face to face after a period of automated messages, they'll be more engaged in the program overall, so it all depends on the nature of what you're building.
It's probably a good idea to ask your users during any testing to get a ballpark, and then aim to refine it, by watching adherence/dropout, after your first beta launch.