I think it depends... just like with cis girls, some people develop fast and early, some slow and steady. (And final size doesn't seem to correlate with speed of growth at all.)
I actually did start HRT and end up 42DDD at a bit over four years, though I may not be that person mentioned; however, I'd say my growth *definitely* slowed a lot over time. I was 42D at the end of the first year, after all. Also, in my experience and based on what the puberty guides for girls say, the initial development gives way to a much slower "fat distribution" stage that's sort of the capstone of breast growth; it's not as obvious and takes longer than breast tissue to arrive, but is what gives breasts that pillowy softness when fully matured. I'm fairly sure *that* stage is still going on for me, albeit very slowly; for my wife, it lasted basically 10 years - from age 15 to 25 - and she was a 36C at 15 and 36J at 25, so it CAN add a lot!
(My mother is 34A and my sister 36D, but my paternal grandmother was apparently very busty. Genetics are weird.)
