Prompts are governed by an Engagement Profile, which allows you to determine how often and on which days learners should be prompted.  You have both an account-wide default engagement profile, and you can also set overrides at the group or user level.

Currently engagement profiles can only be set for the number of prompts per day.  In the future, there will be options to instead target for mastery by a certain date, or allow the learner to set their own prompting schedule.