It depends.
There are some newsletters I’ve unsubscribed from because they emailed too often—generally once a week (sometimes more). The problem wasn’t so much the frequency as the fact that the email didn’t have any content I found useful. Often, it was little more than links to books by other authors in the same genre, and the author said s/he hadn’t read the books. Why would someone advertise or endorse a book they hadn’t read? It seems like a recipe for disaster.
I don’t mind more frequent emails when there is something to say e.g. a book is available on preorder, or there a book is on sale. I especially don’t want to miss out on sale books, and a lot of sales only last a few days. Readers might miss out of the author waited for their regular monthly or even weekly newsletter.
But in general, less is more.
I’m happy to receive a newsletter once a month, or more often if there is a good reason. Otherwise, I’m likely to either unsubscribe or (worse) delete without reading.
