I don't have numbers with which to quantify, but if I lay off the weights for a while (like right now, as I had a broken back fixed 8 weeks ago), and then I start up again, for the first two or three weeks my gains are massive. I mean seemingly impossible in scope. My friends have all commented on this, to the tune of expressing disbelief even thought they're looking right at me. They only say that I'm one of those rare cases. There always exists extremes on any scale, so I would say there are a handful of people who can put on prodigious amounts of muscle in a short period of time.
That's not to say the poster who claims rapid growth actually attained it in the time frame he mentioned (I have no reason to either disbelieve him or believe him), but there must be a fringe of people who can do it. I suppose I'm close to that fringe, along with the "lunatic fringe" that has been immortalized in song, but that's another post.
I lose it equally quickly. I'm no expert in physiology by any means, but when I start lifting, I'm seeing and feeling real muscle developing, and the fat and water burning off. After 3 or 4 weeks I reach a plateau that's mighty hard to overcome, but on the plateau I look pretty darn good.
Submitted FWIW, which is probably not a heck of a lot since it's simply anecdotal.