I didn't found how check and change the default arp cache timeout on Linux. I using debian with kernel 3.x (but I supposed it same with older kernels)
I acknowledge that as I write this, this is a three year old question. But I came across this question while researching the same subject, and in the process of corroborating watchmansky's answer (https://serverfault.com/a/684381/188907), I learned a bit more about the situation, at least as it exists today.
According to https://linux.die.net/man/7/arp, the setting
effects how often the ARP cache is checked for stale entries. (Or garbage collected, hence the "gc_" at the start of the setting name.)
Meanwhile, the value
actually controls how long an ARP cache entry is valid, and it defaults to 30000 milliseconds. But each new ARP cache entry will actually receive a time to live value randomly set somewhere between
base_reachable_time_ms / 2 and 3*base_reachable_time_ms / 2*.
This means each new cached ARP entry will have a starting timeout between 15 and 45 seconds, unless the value of
base_reachable_time_ms is changed.
This assumes that the current timeout value for a cached ARP entry is validated before use, and that the garbage collection rate does not influence the effective validity of the cache entries.
(*Confirmed by reading the code at https://elixir.bootlin.com/linux/v4.17.11/source/net/core/neighbour.c#L115)