Currently Cardinal will email you immediately any time a new card has been shared with you.
When a card you are watching has been updated, Cardinal waits 4 hours to notify you about the update. If you view the update before it's time for Cardinal to send you the notification, Cardinal will skip the notification email. We send notifications for updates in batches, so you'll receive one email with all pending notifications.
It is on our roadmap to give you more control over email notifications.