I started HRT around mid-October 2013, and I was passing half-time by mid-January 2014. The turning point there was when I got my eyebrows waxed. xD That really totally changed everything for some reason. It's tricky to narrow it all down though, but I think earlier in January I got my first ma'am since starting to transition, and that was with guy clothes on and carrying a small purse (and I had longish hair). And that was even talking like a guy. Surprised me a lot, but it happened. After that it was hit or miss, but there was a point where I realized I passed as long as I didn't say anything, so it was only my voice that was giving me away. If I didn't talk or if I faked a feminine voice, I passed consistently. By March, I went full-time and haven't been sir'd since. I've gotten better with my voice too, and now it's full-time as well. It all happened pretty quickly but I think that January and February where were I shifted into 'male-fail', and actually had a difficult time with deciding to go out as a guy or a girl because it was about as much effort to look like a guy at that point. That was actually probably the worst part, because I hated the inconsistency, and that's probably what drove me to go full-time.
Now, I've only recently started going out with makeup on. Before that, I'd be plain-faced and passed just fine. And passing to me is just getting ma'am'd consistently by everyone and not getting any weird looks. I blend in just fine. I've traveled across the state and been to quite a lot of places, been to various stores shopping for women's clothing, gotten my nails done twice (in a room full of women, actually talking with them), been to the beach, used various women's restrooms, and went to a theme-park with hundreds of people around me. In all these places, I've never had any confrontations, weird looks, never got sir'd, never had any problems. In fact, I've even gotten some compliments.

Astounds me every time that I pass in all these instances.. Heh, and at the theme-park, I went on rollercoasters and saw my pic afterwards. Still passed! xD I was actually quite surprised that I still looked female even making a weird face and fighting g-forces. What I noticed was that my face looked rounded, and appropriately fatty. My makeup was subtle so couldn't really be seen in those pics, so I'd say it was all just the shape of my face and maybe some undefinable quality. Part of this I'm sure is due to HRT, and part of it is probably genetics and luck. But yes, I do pass fine without makeup and probably in guy clothes as well. And if I held my hand over my crotch, I could probably pass nakey too. xD
I have wondered about all this though. I think people pick up on different things, and hopefully you have enough of those things to consistently pass. Body, face, makeup, purse, hair, clothes, walk, talk, gestures and other body language, scent?, attitude, confidence.. there are a lot of things one can pick up on, and go by to gender someone. Really, I sometimes feel I get gendered as female because of simple things, like carrying a purse. As long as you don't go out of your way to seem masculine, I think obviously feminine things like that are enough to pass with. Of course, I could be wrong. Please don't go testing my theory if you aren't confident.