It took six months before I couldn't successfully convince *anybody* who hadn't known me before that I was a guy, even in men's clothes and flashing male ID (they said it wasn't mine!). It was only about 3 months until was a 42C and the facial changes were enough that I'd have to be in loose men's clothes and trying pretty hard to pass as male... that's what accelerated the plans to come out at work.
Telling my employer and coworkers was one of the scariest things I've ever done, but it went fine; in fact, everyone was super supportive. (I'd already come out to friends and family pre-HRT.) They changed my name before I did it legally, and made it clear that anyone who was rude to me would be ordered to knock it off.
If you're going to have to tell everyone eventually, you may as well start nerving yourself to do it as soon as you can, and the reward is getting to live as yourself.

I know that's easier said than done, and I was literally sick with terror before I went through with it, but you're probably stronger than you think.