Gaining

I know it would be extremely unhealthy, but how fast could someone gain fat? I've heard of 30 or 40 pounds in a month, but is this realistic?

I have put on five pounds in two days. I'm old, so I can very easily. I did it eating 2500-3000 calories at each meal. I was even fairly active and still put on weight.
Your metabolism really dictates how much you can gain and how fast.