Once you get to know each other better, I'd ask yourself if you would still be attracted to her and still find her compatible with you and with your other interests if she didn't smoke. If the answer is yes, and you find yourself wanting to be exclusive with her, then the time is probably right for you to open up to her about your little kink. The longer you wait, the more likely you are to stay in the closet, and the greater the chance that she will freak out because you've been hiding your interest in her smoking from her...

My $0.02.

;)