How often should I be sending emails?

It depends.

Maybe you create a lot of content and you share it with your list every morning at 7 a.m. Your list could love that you are consistently sending them content. 

Or maybe your list doesn't want to hear from you more than once a month. For example, if you were a dentist, your list might feel nagged if you were sending them flossing tips more than once a month. 

The bottom line: Send email as often as people want to hear from you. You'll only find that out by running tests that help you determine how much is too much.