The most typical answer you hear is 5 years for a full beard.
You also hear that it is genetics. Both sides of my family had no problems with growing beards and my mother said that my father was hairy all over at age 18. Here I am 8 months on T (the majority of it lower dose, though), and I only get hair on my upper lip (I shave it every week. I assume it would only be a pedo stache).
Also, except for a bit of hair on my ankles, I have had no hair growth, anywhere. So genetics may determine how hairy we eventually become, but maybe that doesn't mean how quickly one gets hair is in the genetics. I haven't sought out any research on that, however.
I actually emailed my T provider yesterday asking to increase my dose due to no hair. However, it is not a huge deal to me because I would still be shaving all the time anyway, as hair growing in here and there on my face isn't something I could have at my office job. Until I have something solid growing in, I will just have to shave.
I don't see myself rocking the "neck beard" anyway. To me, personally, that screams adolescence and since I am 45 years old, I can't really be doing that.