This is how long a name server should wait to retry an attempt to get fresh zone data from the primary name server if the first attempt should fail. Somewhere between 120 and 7200 should be okay.
A shorter value indicates that systems will retry faster if they encounter a failure. A longer value indicates that systems will wait a longer period of time before retrying.
Retry: If a secondary was unable to contact the primary at the
last refresh, wait the retry value before trying again. This
value isn't as important as others, unless the secondary is on
a distant network from the primary or the primary is more
prone to outages. It's typically some fraction of the refresh