Snowboarderchic is right about confidence, and ya know what they say. The greatest love of all is to love yourself! To answer your question, it took maybe...5-6 months? I was ECSTATIC the first time it happened. I was 18 years old, dressed as a boy and at a jewelry counter with my mom and we talked to the lady. Afterwards my mom was like "Uh, did you hear her? She called you a girl." Next it came with people telling me I'm in the wrong bathroom, being called ladies, etc etc. At 16 months it was too much to handle. Coming out of bathroms, if a guy was going in, he would pretty much ALWAYS do a heel-face turn. It's sort of like a snowball effect, letting the estrogen and anti-androgen do their job while your hair grows longer and you being to dress a bit more androgynous.
