There are no real hard and fast rules here. Different schedules work differently for different artists, based both on your vibe, interest in creating content, and how your audience responds.

There is an old marketing adage that says "your sales will be in direct proportion to the number of times you ask for the sale". To that end, there are artists that email every day, and while this approach gets them the most flack, it also tends to generate the most money. This only works when you are driving a lot of daily leads though (as in dozens or even hundreds).

However, I am not personally comfortable with that, or interested in creating that much content, so I tend to think that once every week or two is good. And then yes, fit in a quarterly promotion or so.

What you should write about is also largely up to you. The job description of the modern musician is no longer to just make music. You are now a channel. How are you going to keep your audience entertained? Most people take a straight forward approach and just try to share insight into their career, in an entertaining way. But the people who really shine are the people that find some additional angle that really makes the content meaningful.

For example, Amanda Palmer tends to be somewhat political and is always making bold artistic statements. By following her channel you go along for that ride and she makes you think and feel, while being shocking enough at times to keep you wondering what she will do next.

By contrast, an alt country artist could be writing about (or making videos) from the road. It's always about giving your audience what they want. What would make a metal fan tune in every week or two and read or watch something from an indie metal artist?

You could literally make a list, and then see which items on your list were most appealing.

\m/