A cup in a few months is frequent.
The thing is an A cup is like 6-10 time less volume than a B cup,
so getting to A is much easier than getting to B, and C and so on.
Its a half sphere, the longer the projection, the bigger the volume to fill.
That's why people think they've stalled when in fact they're still growing, its just not as spectacular than 0 to something, its something to something more.