I honestly don't think HRT alone is going to make anyone "passable". There is just sooooooooooooooooo much more to it than that. It's just not as easy as popping a few pills, waiting a year and shazzam you're a passable, indistinguishable from genetic, female. It takes works. Hair, make-up, clothes, voice, laser/electrolysis, mannerisms, body movement, facial expression, language, speech patterns, and on and on... are all things I think are way more important in making someone "passable".
I've been full-time 24/7 for about the last 3 years, at work, home, everywhere, and have only just started HRT for the last 7 of those 36 months. Don't wait around for HRT to work some sort of magic-overnight-gender-transformation, cause they don't. They'll help some, that's for sure. And the younger, the better the results. But there's so many other things to work on that will help you "pass" more than HRT.
For the record, I hate the word "passable". It carries the connotation that we (MTF or FTM) are trying to pass ourselves off as something we are not, or trying to fool, lie to, or trick others, which is definitely not the case.