I usually back up when Action Centre starts to mention it. (I have a habit of clicking the Action Centre notification icon in then notification area.)
(Also, if you're using Windows Vista and newer, take advantage of Shadow Copies/Previous Versions/File History. This is also the reason why you shouldn't disable System Protection/System Restore unless absolutely necessary. Sometimes, you can find old versions of a given file or folder without having an explicit backup of your user data.)
(One fun thing about Volume Shadow Copy: you can make a system image of a live system that way!)