Keeping an Office ProPlus License Active
For Microsoft Office 365 with a Named-User license, you need to connect to the Internet at least once every 30 days. Every time you launch an Office application while connected to the Internet, your computer connects to Microsoft's Activation and Validation Service to verify the license status and extend the product activation. As long as you launch a Microsoft 365 application while connected to the Internet at least once every 30 days, your applications will remain fully functional. If the computer goes offline for more than 30 days, Office enters reduced functionality mode until the next time a connection can be made.
In reduced functionality mode, Office 365 remains installed on the computer, but you can only view and print their documents. All features for editing or creating new documents are disabled, and the user sees a message stating "M365-product-deactivated. To reactivate your license, click Sign In, then sign in using your NetID@cornell.edu address and its password."
Allowing your computer to connect to the Internet and launching an Office application at least once a month will prevent this from occurring.
For more information, see Microsoft's Overview of Licensing and Activation on Office 365.