We recommend dictation twice a week for 4th-12th grade.
There isn’t a set-in-stone number of required written narrations. Fourth graders who are fluent in oral narration begin with one written narration per week. Once the student is comfortable with that one written narration, then another one can be added. By middle school and high school, your student may be doing 3-5 or more written narrations each week. It is really up to you how many you want to require. One important thing to remember is to make sure you balance the number of written and oral narrations in a week. Even high school students should continue with some of their narrations done orally.