The easy answer is when you are looking for a new job. But that is too simple. Your résumé is not just the introductory document for a new employer; it can also be used for other equally important purposes.
I tell potential candidates that they should update their résumé at least once every six months. In addition it should be updated before every performance review.
Why?; well first off it insures that you are always ready to respond should your dream job become available in the open market. It is also an easy way to keep yourself current on your own achievements (or lack thereof) and it’s an effective way to keep you focused.
The update before a performance review is fairly obvious. If you have done the work of updating your résumé, you have effectively prepared for you performance review ( and are ready in case it does not go well :)