Exactly when your University software licence will be deactivated is difficult to say because is triggered by your staff or student record being deactivated by the University. This normally happens shortly after you leave Cambridge.
Once your record has been deactivated by HR (staff) or in CamSIS (students), your user account will no longer be synched with Microsoft's Azure Active Directory, at which point you will no longer be able to authenticate to Microsoft's portal using your University login, and you will not be able to run your local software installations.
You are responsible for managing your software licences once you leave Cambridge.
Office 365 products
Files you have created with the software will be still be available to you as 'read only' using the software that you have installed under the University's licence. If you wish to edit them, you can do so using any licensed copy of the Microsoft software.