For unmanaged there is no configurable option on the GUI but try this reghack:
HKLM\Software\Wow6432Node\Symantec\Symantec Endpoint Protection\AV\AdministratorOnly\General
Edit the WarnAfterDays key
https://www-secure.symantec.com/connect/forums/change-definition-out-date-warning-unmanaged-client#comment-10165631