In my experience as a résumé writer, 99% of the clients who reach out to me are in a huge rush to update their résumé due to an unexpected layoff, a sudden dismissal or seeing an ad for their dream job to which they must respond within 24 hours. Wouldn't it be far easier and more sensible to proactively keep one's self in a state of readiness for whatever turn life might take by keeping one's credentials current and available? After all, we take preventive health measures such as annual physicals and yearly dental checkups. We take our vehicles in for routine maintenance. Why not take the same approach to our careers and valued source of requisite income?
What would my advice be as an experienced recruiter and résumé writer? Update your résumé no less than once a year so that you'll be "résumé ready" if disaster strikes or, more happily, opportunity knocks.