Anytime you like.
If you've exceeded the trial date (or accidentally deleted the C:Windows.old files through running the Disk Cleanup tool) either of which would make the rollback option nonfunctional you can always do what is called a Factory Reset by clicking "Settings/Update and Security/Recovery/Reset this PC".
Make sure you have saved any new photos or other personal files as well as all your links to a flash drive or somewhere and then click "Get Started" and then "Restore Factory Settings".
Yes, there is an option for "keeping your personal files" offered. In Windows 8.1 this was known as a "Refresh" but remember that option was (and still is) designed to work best when keeping the same Operating System or Windows Platform as it is called.
You want to restore an older version of Windows so it's best to clean it out thoroughly and then let it reinstall Windows 7,