The timer that determines the frequency between packets sent to verify that the HA functionality on the other HA firewall is operational is the Hello Interval. The Hello Interval is the interval in milliseconds between hello packets that are sent to check the HA status of the peer firewall. The default value for the Hello Interval is 8000 ms for all platforms, and the range is 8000-60000 ms. If the firewall does not receive a hello packet from its peer within the specified interval, it will declare the peer as failed and initiate a failover12. References: HA Timers, Layer 3 High Availability with Optimal Failover Times Best Practices