I highly recommend reading newsletter ninja, it's a quick everything guide to newsletters
Mine are short for the most part.
I send mine out weekly on Tuesdays.
The first Tuesday of the month I send one out that is like an update of me and any news specific to my writing. So announcements giveaways, sample chapter links etc.
The other three weeks are newsletter swaps and campaigns from bookfunnel and story origin.
Every newsletter I send has a question in it where I ask my subscribers to respond to me and if they do I go back and forth with them.
I always include my social media links
I always include whitelisting information as well
I suggest signing up for your favorite authors' newsletters to see what they're doing.
I honestly don't spend a lot of time a week setting this up. Maybe half an hour a week spread out depending on how the sign ups for swaps cluster