For Microsoft Office ProPlus, each day, or each time the user logs on to their computer and launches an Office application, the computer connects to Microsoft's Activation and Validation Service to verify the license status and extend the product key. As long as the computer can connect to the Internet at least once every 30 days, Office remains 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 ProPlus remains installed on the computer, but users can only view and print their documents. All features for editing or creating new documents are disabled, and the user sees a message like the following:
O365-product-deactivated
To reactivate your license, click Sign In, then sign in using your NetID@cornell.edu address and its password.

Turning on your computer, allowing it 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 ProPlus.