The simple answer:
Once a week: that’s the general rule for B2C. Once a month for B2B. Those are frequencies that most people are fine with.
The nuanced answer:
It depends. Ask yourself: what’s the true value of my messages for my customers? Depending on the content of your emails, you should determine an appropriate frequency. Value can be created with extra information, education, entertainment, special offers, etc. Keep in mind that everything you send out is a reflection of your business and influences your customers’ perception: positively, as well as negatively.
In other words: the frequency is determined by relevancy. Stop searching for that magic number in market research, but focus on what your subscribers are telling you through email metrics. Nowadays, email programs can adjust the frequency to ebb and flow with your subscribers’ changing needs. Or why not even let them determine it for themselves?
The real challenge is to remain top of mind and still stir up excitement for every email to come.