Throughout "The Science and Fine Art of Fasting" Dr. Shelton emphasizes the necessity of fasting until the return of true hunger. He says that too many people quit just when their body is about to make a final push to detox.
Here is a quote from the end of the book:
"I am not content with half-way measures. I have seen them fail too often, where a subsequent fast brought speedy results, to be fooled by the claims of the inexperienced and poorly informed. In many cases, failure has been so marked that the patient has been permanently soured upon the whole idea of fasting."