Every few months. Alot can change in a short period of time. Its always a disappointment when you're expecting one thing and get surprised by another behind the door.
I'd say once a year. More often if major physical changes have occured since your last photo shoot.
Think of it as marketing and product placement. If a customer has seen the same product on the shelf for 10 straight visits to the store and passes it by without purchasing it, do you think they will purchase it on the 11th trip. Thus, stores tinker with product placement and marketing execs come up with new packaging and creative advertisements to put a fresh spin on the their products and reintroduce them to the public to spur product sales. It's that new look that gets that customer who was on the fence to take a second look and ultimately purchase the product.
Think McDonald's, Burger King, etc. These companies require their franchisees to update their buildings periodically and they add new items to the menu (some seasonal) and they have marketing campaigns for their traditional items. All of this is done to keep the repeat customers coming back and to get new customers in the door.
Thus, your original photos may get me in the door the first time. A new set of photos could be the driving force that gets me to return a second time and a third and...
I would presume if there are any substantial and/or material changes. A new hair color, cosmetic changes, etc. In any event, new photographs on a regular basis allows for consistent market awareness. The product may be singular like Coca Cola, but automakers make new models annually. New photos show your commitment to yourself. TV