Does anybody know what the ARP cache timeout is for macOS Sierra?
There's no standard time out value for routes stored in an arp table.  For instance, on Cisco gear, it's 4 hours.

The arp utility that macOS uses is the one from BSD and it's time out is 20 minutes if the host has been validated. Static entries don't time out, but can be over written by network traffic.  Permanent entries also do not time out and cannot be over written.