How often should you send a newsletter?
As often as you can sustain with genuinely useful content; for most newsletters that is weekly or biweekly. Consistency beats frequency: a reliable biweekly email builds more than an unpredictable weekly one.
There is no magic frequency, but there is a floor and a ceiling. Mail less than monthly and people forget they ever subscribed, which shows up as unsubscribes and spam complaints. Mail daily without every email earning its place and you train people to ignore you. The healthy middle for most newsletters: weekly or biweekly, at a fixed moment.
The real question is what you can sustain. A newsletter that ships weekly for three months and then goes quiet does more damage than one that calmly arrives every two weeks for years. Pick the rhythm you are certain fits around your work, and increase only when it feels easy.
Watch your own numbers to adjust: rising unsubscribes after a frequency bump are a clear signal. And set the expectation at signup ("every Friday in your inbox"); expectations that match reality produce loyal readers.