After 30 days, Microsoft Office 365 ProPlus will go into reduced functionality mode. When this happens, users will be able to open files but they won't be able to edit them.
As part of the installation process, Office 365 ProPlus communicates with the Office Licensing Service and the Activation and Validation Service to obtain and activate a product key. Each day, or each time the user logs on to their computer, the computer connects to the 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.
Reference:
https://docs.microsoft.com/en-us/deployoffice/overview-of-licensing-and-activation-in-office-365-proplus