I think it all depends on the lady:
1. Some girls use the same photos for years! Ad: I'm 22 going on 40... LOL!
2. Some girls update photos every time they travel to a new city - cell phone snaps in the mirror are very easy to update.
3. I recommend to my clients / (ladies who value high quality photos / marketing and invest in their business) with pro photos should update either when they change their look (hair color, weight, tour, new menu, etc) or when it's been a while since they last posted new pro photos example: every 2-4 months.
SIDE NOTE: I think most guys appreciate when a lady post new photos - it's kinda like Christmas - you had them on your list and NEW photos reminds Santa how naughty you're willing to be to get that new toy!