Command Description:

To define how much time should elapse before a secondary line status changes after a primary line status has changed, use the backup delay interface configuration command. Use the no form of this command to return to the default, so that as soon as the primary fails, the secondary is immediately brought up without delay.

We recommend that you enable some delay before activating and deactivating a secondary line.

Example :
The following example sets a 10-second delay on deactivating the secondary line. However, interface gigabitEthernet0/1 is activated immediately.

The enable and disable backup delay must be 0.
!
Router(config)#interface gigabitEthernet0/1
Router(config-if)#backup interface gigabitEthernet0/2
Router(config-if)#backup delay 0 10

which means if the primary link is back up again, the secondary should wait for 10 seconds before switching the traffic to the primary.

Once you activate the command , immediately the Back up interface will be in Standby mode and once the primary interface is back up, it should wait for whatever seconds configured as it will preempt to check if the interface is alive and waits for the specified amount before the traffic is back to the primary interface.