The answer to this question depends a lot on what your goals are. If you're just starting out and have no idea what you want to accomplish with your business, I'd say once a month.
These newsletters can keep you up to date on industry news and trends, as well as how to improve your business.
However, if you already know what you want to do and want to build a brand, I would suggest sending out a newsletter twice a week. This way, you can provide your clients with valuable information while keeping them up-to-date on what's happening in the industry.
I usually send out my newsletters several times a week, and for one reason only:
I don't want email readers to forget about me!
One must always keep in mind that, on average, an email recipient receives at least 10–20 emails per day. Some statistics even speak of 100 emails or more per day. That is pure madness!
If you send your newsletter only once a month, it means that the recipient has received hundreds or thousands of emails, none of which were from you. You would be more and more forgotten.
However, if you send the newsletter more frequently, then your emails and, of course, your name and brand will always be fresh in the recipient's mind, and you will remain a known person or company.
A newsletter should be sent as often as possible (but not more often than every other day) for this reason alone, so that you remain in the recipients' minds and they remember you.
