This article explains how many notifications are sent out once an encrypted email was sent to an external recipient.
Default configuration: First notification is sent to the recipient after the encrypted email was sent out. Second reminder is sent after 15 days and the last reminder is sent a day before the secure email expires. Encrypted emails are expired after 30 days it was originally sent out.
Customization: A new case is required to be opened and Support will reach out to the Encryption provider to have the notification frequency/timing modified.