I've told every girlfriend I've ever had within a week of seeing them. I want to give them the chance to leave if they feel they can't handle the fact. None have left me.
The first one was proud because she realized how seriously I take my training, eating, and results, as she had been a competitive athlete in several sports.
Others were indifferent.
My current girlfriend was worried and said I'd suffer from [fill in the blown out of proportion side effects here], but I took her down to the doctor's office with me for a physical exam, and I came out in perfect health. Better than hers. lol I have blood tests every so often, too, to keep internal tabs on things.
It depends on the kinds of girls you involve yourself with, how intelligently you are able to communicate, how you behave, and more.