I suppose it could vary but I think mine went on about 2 months. However I had been through more than just the gastric sleeve so there was a lot of stress on my body. It does look like its starting to grow back in, which is easy to tell on me because that hair is grey, lol.
Mine started to fall out in literal handfuls at 3 months. The nutritionist told me it could continue to happen up to 9 months however 6 months seems to be the average.