This policy setting allows you to define the number of days that must pass before virus definitions are considered out of date. If definitions are determined to be out of date this state may trigger several additional actions including falling back to an alternative update source or displaying a warning icon in the user interface. By default this value is set to 14 days.If you enable this setting virus definitions will be considered out of date after the number of days specified have passed without an update.If you disable or do not configure this setting virus definitions will be considered out of date after the default number of days have passed without an update.
Windows Components\Windows Defender\Signature Updates
At least Windows Server 2012 Windows 8 or Windows RT
HKLM\Software\Policies\Microsoft\Windows Defender\Signature Updates!AVSignatureDue; HKLM\Software\Policies\Microsoft\Windows Defender\Signature Updates!AVSignatureDue