Every Alarm Zone on the Bridge has a "Alarm & Video Event Interval" option.  The default timer for this setting is 20 seconds.  This value can be adjusted between 5 and 3600 seconds.  The setting is found on the Alarm tab of the zone settings:

When this Alarm zone is triggered on the Bridge, the Bridge will ignore any additional alarms on this zone for 20 seconds.  This timer is from initial alarm trip of the zone.  If the zone is Normally Open, then the 20-second timer starts when the zone closes.  If the zone is a Normally Closed zone, then the 20-second timer starts when the zone opens.  It the zone is using camera analaytics, then the 20-second timer starts when the analytic trigger occuers.   If the zone is triggered multiple times within the 20-second timer, only the initial trigger will create the alarm and video event. After the 20-second time is completed, any new triggers will start the sequence over again, with the first trigger creating the alarm and video event.