It depends on the person. Some people feel it before others, and some people grow more than others and can grow for a long time after. 3 years into HRT my breasts really hurt badly, still, and are still growing! Yet, I know people who haven't much growth after two years. *shrugs* genetics, age, and a bunch of other factors come into play. And, believe it or not, I've seen in some cases where younger doesn't always mean faster or bigger breasts -- I've seen the opposite in some cases! Weight and breast size can also come into play, I know bigger women on HRT seem to get pretty big breasts sometimes, and really skinny women don't get that much.
Anyway, congrats on your new growths.
--natalie