After your 180 day license expires, you are immediately locked out of all source control operations with an error like "The request failed with HTTP status 403: TF30072: The Team Foundation Server trial period has expired or its license is otherwise invalid. Install a licensed edition of Team Foundation Server to continue."
If you need to grab your source one last time (to export into another version control system, for example) - set the server clock back. It takes effect immediately.