Update your resume at least once a year whether you are looking for a new job or not.
Reviewing and updating your resume yearly gives you the opportunity to strip out past successes and add your most recent accomplishments while the details (i.e. numbers) and results are still fresh in your mind.
Even more importantly, it gives you the opportunity to evaluate and assess your progress in meeting your career goals and will help keep you focused and on target.
One last to reason do so is, as Jo Lynne Lockley of Chefs Professional Agency points out:
Sometimes fate smiles only for a second, and you will need to respond. For some reason this window always happens when you are in the middle of an 80 hour week. Having a good resume on file makes it possible for you to send your information without slogging through text your mind is too tired to take in. Anytime there is a change in your job status, do your resume changes within a week.