Pretty much after the first time. I was really really really nervous and was primed for someone to look at me weird or clock me. It didn't happen, nobody even looked twice at me. Everyone just went about their business and even gendered me female pretty much 100% of the time. So I no longer felt like I was constantly the center of attention. Realizing that no one is sizing you up or evaluating you constantly (except for yourself, of course) goes a long way towards feeling less self-conscious.