The 'Recurrence Interval' of the Archive/Purge facility determines how often the Archive/Purge rule is run during the chosen workshift Schedule.
An Archive and Purge Rule fires at the specified start time of the Schedule.
For example, in the case of the out-of-the-box Non-Business-Hours schedule, the workshift is defined, for weekdays, as 5 pm to 8 am. When a rule using this schedule is active, it fires at 5 pm each weekday. It then gets fired at every occurrence of the 'recurrence interval' within the schedule. In the case of the non-business-hours schedule this occurs until 8 am in the morning.
Note that in the case of a 24 hour schedule, when you restart the Service Desk Manager services, the rules fire again even though the recurrence interval has not yet been met.