About five days (also when I had to cup them going down stairs, a lesson I learned painfully). But I had some gynecomastia at puberty, including breast buds that eventually faded away, so I suspect I had a head start on development and E mostly just perked up and out what was already there.
It was about another two months before I went up an actual cup size from more or less my starting point (what would fit in a 38A bra, though I didn't wear a bra yet).
(4 years in, either 38DD in expensive bras or 42DDD in cheaper ones, still growing but very slowly.)