I've been on HRT for about 5 months now... the growth for the first month or so it was just some fat over my chest, which I've never actually had, always been skin and bone there. After I don't know, maybe a month, it was a conic "bump" under the nipple, with a pins and needle sensation as it was developing. That happened for about 2-3 weeks, then stayed as a bump, and I was able to wear just a T-shirt without drawing attention.
More recently however... I had a more round mass: Nothing to put into a bra, but I felt more comfortable hiding it with a tight A-Shirt under my T. That lasted for about a month. I don't have any actual A-cups to test to see if I fill, but I developed pretty quickly after that to wearing Wal-Mart's 36B cups (couldn't find A's in 36). They don't have cups, but rather hold me up, to try to still hide it... which I do under an A-shirt still. XD (I hide because I still live in the bible belt)
I have a K-Mart "Pink" something brand bra in 36B, and don't nearly fill it, but since I'm not half a year in yet, I'd say a B cup is easily developed, unless you just stay a skinny Minnie. I'm only a 36 because I'm over 6 foot tall, most of it was bone, until some fat distributed there. As everyone else says, YMMV, but I think a B cup is basically minimum standard issue, from what a majority of other things I've read state.