The settings that determine how often a quarantine email is sent are specified at Setup / Users -> Mail -> Quarantine Configuration. These settings can be applied on a global, domain or user level and it’s possible to apply custom settings on each level.
To give a user the ability to change their quarantine email settings, tick “allow users to modify their own settings”.
With this enabled, when the user logs in they can then make associated changes to their quarantine settings via “My Settings”.
Note: a quarantine report email is only sent if the user has one or more messages in their quarantine.