We continuously experiment with reminders to find the best time to sent them, but currently they are sent out after 4 days to all recipients who have not listened to our downloaded at least 1 track on the campaign. We want to eventually allow users to adjust this setting themselves in the future and to automatically throttle how often a recipient receives emails (while prioritising new promo sends). It is not available at this time.
To turn reminders on we recommend you do this during the campaign creation. The option can be found at the bottom of the campaign creator.