If you are under the age of 15, I would say get new shots every six months… your look tends to change drastically in your young years. Once you are over 21 I would go with once per year, unless you do some drastic change to your look. Like a face lift, totally different hair style, etc…
You want your head shot to be the best representation of you, and who you are!
The head shot is, aside from you acting ability, is your number one tool in getting you an audition. The better your shot is the better chance you have of getting seen. Do not sell yourself short on this point.
For the full details on everything you need to know about this topic go to this site: Head Shots 101
-Frank
