My wife (who is not a member here) was convinced she was imagining it until she tried to run down a flight of stairs at 5 days. 😉 She went from slightly visible "male" breasts to an A cup within a week, and was fitting a 40B by the end of the month. She was very lucky she had plans in place with her employer to transition within a couple months, because by then she was actually having to bind with a compression bra to go to work. When she tells this story, occasionally another woman says she thought she was the only one... So some people do see what seem like impossibly quick changes, at least in some respects. Incidentally, the one common thread I've noticed in all of those stories is that the women did have a bit of breast development even before HRT, and their bodies seemed ready to "jump-start" as soon as they took estrogen.
(Now, at almost three years on estrogen [only], she's a 42DDD. Growth is still continuing, too, albeit obviously not as dramatically as in the beginning.)
