As this is written by an email marketing service provider you would expect the answer to be “as often as possible”.
The correct answer however would be whenever you have something relevant to say. Too many email campaigns are sent once a month or once a week in a rigid fashion. Why is this?
Probably because the website says “Sign up for our weekly newsletter.”
Or maybe it is because it is thought:
“The more emails I send the more orders I get”
Marketers that base their email marketing calendar on this approach are seeing list fatigue – a massive drop off in response over time as recipients get bored with receiving irrelevant information in their inbox.
It of course works the other way as well – don’t wait until your set day of the week or month if you have something relevant to say but get it into your recipient’s inboxes as soon as possible.
If it is relevant your customers will read it!