A name server will no longer consider itself Authoritative if it hasn't been able to refresh the zone data in the time limit declared in this value.
MxToolBox will issue a warning if your value is less than 2 weeks or more than 4 weeks, which are suggested values.
Why you need authoritative servers
Each DNS host has their own interface, but you are looking for either a setting labeled Expire Value or you might have to enter your SOA details manually. If you have to enter your SOA then the Expire value will be second to last number in the SOA.
Your DNS records are hosted on two or more DNS servers that are supposed to be in regular contact with each other so that they have up to date copies of your DNSrecords. The Expire Value setting tells each slave server how long it is allowed to continue giving out authoritative replies after it has no longer heard from the master server.
RFC 1912 recommends 1209600 - 2419200 seconds (14-28 days).
How long a secondary will still treat its copy of the zone
data as valid if it can't contact the primary.This value
should be greater than how long a major outage would typically
last, and must be greater than the minimum and retry
intervals, to avoid having a secondary expire the data before
it gets a chance to get a new copy.After a zone is expired a
secondary will still continue to try to contact the primary,
but it will no longer provide nameservice for the zone.2-4
weeks are suggested values. [RFC1912]
@ IN SOA nameserver.example.com. postmaster.example.com. (
1; serial number
3600 ; refresh [1h]
600; retry [10m]
1209600 ; expire[14d]
3600 ) ; min TTL [1h]