I recently has to configure a VDOM for a guest network. As there are anywhere between 300 - 500 clients a day, we decided to drop the DHCP lease to 12 hours, as the the netmask is already a /22. This can only be done via the CLI.

Note that the lease-time unit is seconds.

config system dhcp server
    edit 1
        set lease-time 43200
        set default-gateway 192.168.0.1
        set netmask 255.255.252.0
        set interface "port12"
        config ip-range
            edit 1
                set start-ip 192.168.0.5
                set end-ip 192.168.3.254
            next
        end
        set timezone-option specify
        set timezone 64
        set dns-server1 1.1.1.1
        set dns-server2 8.8.8.8
    next
end