If you want to set the ARP cache timeout in Screen OS Firewalls use the following procedure
Procedure to follow
First you need to login in to your juniper firewall with SSH once you logged in run the following command
get config | inc arp
If the output generates nothing, then the ARP age is set to the default value of 20 minutes. To set this timeout value, use the set arp age command, along with the number of seconds. For example, the following command will set the ARP cache timeout to 120 seconds:
set arp age 120
Make sure to save this change by using the save command; else it will be reverted, when the firewall is rebooted.