Writing aubmissions

I’m thinking of setting a writing submission to accompany each discussion topic, to encourage students to write more. Are there any guidelines to how long they should be for each level?

I don’t really understand your question. There already is a writing topic for each lesson. Beginners are to write 50 words per lesson, Intermediate learners write 100 words and Advanced 150 words. Of course, you can ask them to write more but the cost of this additional writing won’t be in the price of the course. They will have to buy additional points which is fine and can be explained to them in your initial conversations if they are unsure.