If you're growing your hair out and actually trying to feminize your appearance, it varies. Could be anywhere from a few months to a couple of years before you start male-failing. For me, the threshold happened at about 13 months of HRT when I started passing without even really trying. With my growing hair combined with my feminizing appearance and my unisex work uniform, people stopped being able to tell which sex I was. People were even switching back and forth between "he" and "she" within the same sentence.
(If you want to see what I looked like at the time, here is a "presentation check" video I made in my work uniform at the EXACT point where people generally started calling me "she" intermittently.)
(This was almost exactly one year ago, by the way. So obviously, that 2nd year on hormones makes a BIG difference. After one year, I was just starting to get to the "androgynous" phase, where now with my hair, body, and mannerisms, I don't know if I could pass as male even if I tried. Maybe if I was wearing a business suit and deliberately walking with my legs apart in a dominant manner and speaking in a super-deep voice? I dunno.)
If you're NOT trying, though, if you keep your hair styled in a stereotypically "male" style and don't try to change your voice at all or feminize your appearance or mannerisms at all, and still wear male clothes, frankly the duration can be more or less indefinite. Hell, I've even heard stories from people who have gotten FFS while still keeping short hair and presenting as male who still could easily keep being accepted as a guy by people. And it makes sense. Even cis women, who never went through a male puberty in the first place, can get "sir"ed when they have short hair and wear masculine clothes. So if you sound the part too, and act like the part, trust me, people can and do easily overlook a little bit of boob development, butt development, and softer features.
Also, this is another thing that I've noticed... especially with the voice thing... people will listen to what you tell them. If you sound like a guy, look somewhat guyish, and have a deep voice, people will readily still accept you as male as long as you tell them that you are male and go by a male name. You can get away with a LOT of s*** in terms of gender-nonconforming features as long as you have confidence in who you are. Even after the point at work where I stopped passing as male to pretty much every single customer, the other poker dealers still accepted me as a guy.
I wouldn't stress out about it too much. There's a LOT of leniency in terms of what people will accept gender-wise when you're still wearing masculine clothes.
