It takes about 7 weeks for the base layer of skin cells to make it up to the surface. If you recall, patients reported starting to get relief from itching after about 2 week. That would be when the bottom layer of cells would have started moving up.

So, a 16 week trial means that there would be about 2 new complete generations of skin cells. That seems like a good length of time to gauge whether it's going to work on a particular patient. Some people might not see the maximum effect after only one layer of new cells.