Hi,
It really depends on how critical your data is. It depends on the number of files you create and how often you create them. If you create new files every day, you might want to back up weekly or even daily. If you occasionally create many files—for example, if you save a lot of digital photos from a birthday party or graduation, back them up right away. It's best to schedule regular, automatic backups so you don't even have to think about it. You can choose to have your files backed up daily, weekly, or monthly. You can also back up manually between automatic backups.
For reference:
Back up and restore: frequently asked questions
http://windows.microsoft.com/en-US/windows-vista/Back-up-and-restore-frequently-asked-questions
Protect your files and PC with Windows 7 Backup
Hope this information is helpful