I say whenever there's a major change like hairstyle, weight gain/loss ect. Or just add an updated pic every 6 to 8 months or so.

It would seem to me that one should always advertise the product with accurate images. To do otherwise risks unsatisfied customers. Only you know what has changed.Good luck.

As others mentioned, if you change your hair color, had a lot of weight gain/loss, etc. you should update your pics.

Your pics are one of the reason why I chose to pick up the phone and call you.

I understand that professional photo sessions can cost a fair penny. They often look amazing, so no complaints there. But, I also enjoy seeing regular posts with candid photos that haven't been photoshopped. I think ladies need to balance the fantasy woman with the everyday expectations. Or, if a lady rather post only professional photos, go for the balanced look. Goddess Jezebel of SD comes to mind immediately of someone who has mastered the balance between expectation and photoshop.

every 1-3 months is enough.

If you havent changed your hair, gained weight, got a new tattoo/piercing, got a new scar or any other significant changes to your appearance there is no real reason to need to update.
Your photos are supposed to be a true respresentation of how you appear NOW
And generally not much changes over 1-3 months .. if yu look the same you dont need  new pics contsantly

But of course knowing that providers often use innacurate pics.. hobbyists get skeptical of any pics older than 3 months .. as well they should!

So be sure to make that your max time.

Hope this help