How long does it take for to completely develop?

So I'm 15 and I've been a 34B since I was about 13.. But then last year my s got bigger and could manage to fit into a 36C, but at the end of that year and till now, I've lost about 2 kgs since I'm trying to badly to lose weight, but my also got smaller.. & I think I'm back to a B36 :s So I'm wondering how long it takes on average for a teenager girl's to fully develop? And how come I'm now kind of a B cup again but not a 34 frame again? And also, will my s change in any other way as I grow up? (That is, if they haven't fully developed yet) Like will they appear more stiff or upright if you know what I mean? Not that they aren't.. But will they?