When should you update your resume? As soon as you accept a new job. Sure, you are probably thinking, I've just got the job, why should I be worried about the next one. The answer is simple, how difficult is it to remember your job responsibilities after you've left a role? Ever try to remember what lead to an accomplishment a year after you received the recognition, or the title's of your direct reports? My point is, always keep your resume up to date.
Too busy to knock the dust off the old resume? Try keeping a running list of the accomplishments you've achieved in your career as well as the projects you've completed.