Answers ranged from "It depends on your audience" through "26 times a year" to "3 times a year".
I think the best answer is to the question "How often?" will be found by following the path of thought that starts with "What is my goal?"
What is YOUR goal in issuing YOUR newsletter?
Write it down, write down the objectives (or sequence of steps you'll take to reach your goal), and you'll find the answer to your frequency question along the way.
I forgot that you wrote all that down when you were planning your newsletter; just take it out an re-read it.