You do not have that choice do you as long as you still have windows.old folder on your drive then you can go back any time that is the backup of your old windows install.
Ok but with windows.old folder i know that, but say you have one motnh to go back w7 is this mean something after one month windows delete the windows.old folder...?
Whole point is your scenario does not exist the only way you will have problems is if YOU delete the windows.old folder or use disk cleanup to delete it. Any way even before updating MS advised users to make backup just in case installation failed. OK I was wrong But already gave advice you should have made a backup.