How often can I shave between treatments for IPL facial hair removal?

How often can I shave between treatments for IPL facial hair removal? I started IPL treatments for my face (upper lip, chin, side burns) in May 2014. I was initially advised to have treatments once every two weeks. I declined, and stuck to once a month. Anyway, 8 months later and while the hairs, particularly on my upper lip are much finer and a little sparser, I still have darker hairs on my chin, under my chin and side burns. My practitioner said that I made a mistake by shaving too often.