How fast you gain the weight back would depend on a lot of things I think. How much you eat and the quality of that food. Also the longer the fast the more likely the body will be absorbing nutrients more easily as well. I have heard of people who were previously unable to gain weight that have done so after a fast.
You can post about dry fasting on the Fasting- General Fasting Support Forum. There is no forum just for dry fasting here but they do discuss it there.