I have done a 3 day dry fast in the past, but it was for spiritual reasons. By the end of the third day, I was DONE! I could barely stand without being so dizzy. There wasn't a hunger issue because I realize when my digestive system is at total rest (including no gum chewing), I lose all desire to eat. I go back to work today so it was not feasible for me to continue the dry fast for I work with children in an elementary school.
If you have the time to rest and not be active, you may want to give it a try. I am not sure what the longest period is, I did read during a 2 day dry fasting weight can be between 8 to 10 pound loss, but it also said its mostly water....
Dry Fasting Weight Loss Rates
Fasting weight loss rates are highest with dry fasting. Using an enema then dry fasting for 2 days can drop, 8 lbs. from the colon, 5 lbs. in water and 2 lbs. in body fat. The detoxification is powerful and you can feel health surging through your body as soon as you go back to juicing but the weight loss is really only 2 lbs.