Take the time every 6 months or so to update your CV. Why not this weekend?
The first reason is that jobs are insecure at the moment, and that will not change any time soon. Being ready to apply for another job as soon as yours is threatened (or you think it might be) means that you can be first in the queue. And finding a new job takes long enough without adding more delay while you write your CV.
The second reason is that it gives an opportunity to review changes over the last 6 months. In particular, how have you built your skills and experience? What steps do you need to take in the next 6 months? How can the changes be reflected in the CV (bearing in mind the CAR principle)?
It is not only the employed to whom this applies. Freelancers and the self employed should consider it as well, since changes in your portfolio are reflected in your market worth.