That's actually hard to quantify as I've been losing weight and body shape changes.
However, I started completely flat with undeveloped nipples last year. I could pinch about an inch near the armpits and barely 1/2 inch near the nipple (too small to even grip). If I remember I was right about 42" bust and 40" underbust.
At 1 year, my underbust is ~38" and the bust floats in 44-46" ranges depending on how much swelling at the time. I routinely use a 38B (base) and 38C (swollen) plus 40C Cacique pushups recently (they run a bit small). I pinch about 3" near the armpit and at the nipple. I have cleavage when lying on my side.
Of course, my nipple development has been the most surprising. From tiny dots in the areola of under 1/2" to these 1" areola with eraser sized nipples of about 1/4" length.
The other changes have been in the lower torso as my figure slowly feminizes too and I find men's clothing rather uncomfortable now (baggy waist and tight bottom).
I know this isn't a number for a "how much" question but answers like 2 or 4 inches would be wildly misrepresentative. I kinda look at this like a 2nd mild puberty as the changes have been profound in some ways. Maybe 2nd year will be a simpler answer but I doubt it.
DJ