Mine hurt pretty quickly into taking estrogen, but then again, I already had enough there to fit an A cup when I started hormones. I guess it depends on the person, and if they have a history of previous breast growth.
Also, as time goes by and your body changes, it might seem like they aren't growing much but they are. They won't just grow out, but grow on the sides and stuff. You're chest can also go down in size (so under your bust can change). When I first started I was 36" around. I recently measured myself at 34". So, I lost inches around, although my breasts continued to grow. I have a larger B cup (I might be hitting a C, it seems, since my cups have started not to fit and really hurt! Having your underwire dig into your breasts because your cups are to small really hurts.)
Other people don't grow much. My friend, who's much skinnier than I am and very beautiful, has just started hitting an A cup after 10 months. They recently adjusted her hormones to a higher dose, so she might experience a little better growth.
But the point is, it really depends on the person. It can also be tricky, and again, with body changes sometimes it can seem they aren't growing when they are.
Also, when I buy bra's I buy new ones (yeah, I know people who go to 2nd hand stores and by old bras -- sorry, bras actually get worn out and stretched out over time, newer is better! Plus... ewww?). I also don't go over board with buying too much because I find that I'm needed to get a new bras come six months time (others it's different, some faster, some slower). A bra should also be confortable when you wear it, not move around or dig in any where.