There is no right answer for this. The frequency of emails would differ from business to business depending on the industry vertical it is a part of. For instance, an AC manufacturer can send at least 3-4 emails a week during summer whereas an eCommerce brand can send out promotional emails almost everyday.
However, by rule of thumb, you should send your highly engaged audience emails more frequently than you send to your lesser engaged audiences. Try to create segments based on engagement activities (opens, clicks, following thru on CTAs, filling forms, sharing etc.) that update constantly. This way you constantly have an idea of who loves getting your emails and can send to them more often.
Talk to our customer success team if you’d like more clarity on this and they can help you out fine-tune ZetaHub to help you figure out your highly engaged (and not so engaged) customers.