Breast buds, visible rounding, and tenderness was day 5 of HRT. I think the nipple thing set in by around 2 weeks, but I didn't realize it was going to be a long-term symptom so I didn't notice exactly when it began.

Nipple/areolae changing color was veeerrry gradual over the course of the first year or so, and it wasn't actually darkening; went from light brown to light pink. Areolae and nipples increasing in size is still happening.
I never had severe pain, though. Occasional mild aching, an few hours of serious tenderness now and again that passed quickly, and my God the ITCHING drove me nuts. But nothing like the pain other women have described.
I'm actually kind of hoping they're done soon (almost 4 years HRT) - a 40C cup in 4 months was cool, but 42DDD is beginning to be too big to easily buy in stores, and even my expensive bras are getting tight. Argh!