Whenever it is needed to maintain your current image
Whether you are Hired from your headshot or Called into an Audition from your headshot - the person in the Headshot must show up for the job or Audition.
So if you cut your hair short OR change the style Drastically or color your hair so the Blonde with long shoulder length Hair is no longer available for work - you need a new photo and need to send that to the potential Employer instead of the old photo
Growing your hair out - You need a New Photo
Use a Glamor shot or Photoshoped Photo and you can not look exactly like that photo EVERY TIME you audition - you need a new photos
Put on 20 lbs (or take off 20) and it shows in your photo - you need a new photo's
Men - Growing Besrds, Mustashes, Van Dykes ETC or shaving them off - you need new photo's
Kids Grow out of Photo looks fast - Many need new Photo's twice a year if their look changes
Many Audlts can go a year or two (some older afdults 3, 4 or even 5 yrs) as long as the Headshot still looks like they do every Day
Best Brian