I'd say, starting from the beginning of August (started prescribed HRT July 31st), by October the very few chest hairs I had went vellus and then away. Now I have literally three hairs that I just pluck every few months, and they are in areas I don't even really care about. Honestly, the rest, when I plucked them out, didn't come back. Now, take into account that my last period of self-medication was from January to the end of June, so that might skew the numbers.
I do have to say, while I would never advocate self-medication and think that anyone that does is toying with their life (seriously, don't do it), doing on-and-off medicating since I was 16 did have enormous benefits - I never had more than about 10 chest hairs, and my facial hair was (now that I realize how fast electrolysis has been going) only maybe 1/10th the thickness as normal males. Epic win!