I dont know if the PA-2020 got some other value but according to the CLI Reference Guide for PANOS 4.1 the default is 1800 seconds:
The following command displays ARP information for the ethernet1/1 interface.
username@hostname> show arp ethernet1/1
maximum of entries supported : 8192
default timeout: 1800 seconds
total ARP entries in table : 0
total ARP entries shown : 0
status: s - static, c - complete, i - incomplete
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