If you use the PC the way I do, never. Others may disagree, but I am comfortable with my methods.
I install the OS then install all my applications. Afterward I update through windows update. Once finished I will then create a system image. If I update the image, it will only be after restoring the backup. That way I know the image will be fresh. I don't feel comfortable creating an image of a system that has a long run time.
Oh and if you are wondering I don't keep personal files on the system partition. All my personal files are stored and back up separately. Managing my OS and data is easier for me this way. And if you ask about data backup frequency, I will say that depends on the type of data. Data that never changes will only need to be backed up once. Data that changes frequently is the only files that need frequent backup schedules.