Which field did you set? There is no specific field for "purge", so i am assuming you set Destroy Instructions.
Destruction happens inside class DestroyDispositionAction.java, which is called from RecordsManagementScript.java.
From taking a look at the code, it looks like only IMMEDIATELY is implemented, the intervals are not.
If you were to implement this, it would be done similarly to modules\records-mgmt\config\alfresco\module\org.alfresco.module.RecordsManagement\script\scheduled\ scheduled_cutoff.js scripts - it is scheduled inside records-mgmt\config\alfresco\module\org.alfresco.module.RecordsManagement\context\script-context.xml.
Hope this is somewhat helpful. The RM module is really a starter kit for implementing the RM functionality, most customers who use it take it and customize it for their needs. If you do end up implementing this functionality, feel free to contribute it.