This directory partition has not been backed up since at least the following number of days.
'Backup latency interval' (days):
It is recommended that you take a backup as often as possible to recover from accidental loss of data. However if you haven't taken a backup since at least the 'backup latency interval' number of days, this message will be logged every day until a backup is taken. You can take a backup of any replica that holds this partition.
By default the 'Backup latency interval' is set to half the 'Tombstone Lifetime Interval'. If you want to change the default 'Backup latency interval', you could do so by adding the following registry key.
'Backup latency interval' (days) registry key:
System\CurrentControlSet\Services\NTDS\Parameters\Backup Latency Threshold (days)
This event is logged when active directory has not been backed up in the configured amount of days.
To backup AD, run NTBackup (or your preferred backup software) and backup the "SystemState".
Once the backup is complete this event will not be logged anymore - unless of course you don't continue to backup.