I have a Windows 7 PC and back up weekly. This seems like a reasonable schedule for file backups, but how often should I create a system image backup; which takes several hours to complete?
Backing up once a week is an excellent start. As for imaging, I would recommend that you do it monthly, and store them on an external drive. Each month, when you create a new image, delete the image from two months before, and keep last month's and the current month (in case something goes wrong, etc redundancy is good).
System Restore points are somewhat similar to images and are extremely useful and much quicker to set and use. Consider it an alternative or for what its worth.