Some lucky folks do not experience soreness but it seems like trans women experience soreness more frequently than cis women. That is not scientific, just what I have noticed from talking about boobs a lot, with a lot of people. Some people develop quite nicely without the added pain.
When does it start? Who knows? It does seem to signify that they are going through a growth spurt. I do not know if they are growing more when they hurt or when they don't. Pain tends to come with growth in one way or another, though. If you go without pain for a while then it returns, it tends to mean there will be more growth soon.
When does the pain end? Again, who knows? For some it never stops. For most, it seems to stop once development has pretty much finished. I am not there yet.
What is the pain like? Well, it hurts extra bad when you press them in, or bump into things. For most, it doesn't seem to hurt at all if you just leave them alone but we growing gals tend to keep pushing them in, just to make sure they are still there. It is a perk of transition. See what I just did there?

I do not remember when the pain started for me exactly, I mean, I didn't realize it was THE pain until it had been there for a while, but it seems like it had started by the 3-6 month mark, and it was consistent for around a year. Lately it has been a bit more on and off.
I take a shot of E every two weeks and the pain seems less when I am running low, only to return shortly after the next shot.
Lately, the time without pain seems to be getting longer and the time of pain is less and less. I hear that may mean that their growth is slowing, or that they are putting their effort into filling in rather than developing glands and growing out.
Fine with me. I am overflowing my C cups and measuring in at over a D at this point. They can slow down. Otherwise, they will become a hassle if they keep growing at this rate for much longer.
Aloha,
Tori