Hi fasting journey. I can only speak about my experience as I still learning about benefits of fasting and all attributes around this subject.
I fell into fasting the first time and went with it up till day 29 and quit upon learning that I had low potassium, a common symptom sometimes depleted by medications such as blood pressure meds or pharmaceutical use of steroids, for my particular situation, is also depleted in fasting.
Lesson Learned: You cannot determine the number of days you will fast as that is determined by your nutrient stores. While excess weight would allow you to continue on a longer fast if your nutrient stores, ie. electrolytes are not adequate, you will need to stop and do appropriate refeeding immediately.
Hypokalemia ( low potassium) can cause muscular weakness, respiratory distress, parlysis and death.
This is not to scare you , but you should likely check in with a doctor or get supervision by phone if you plan an extended fast. I was told an extended fast was anything beyond 10 days.
All people have different biochemical and genetic make-up. How you respond and what your resources are, are different from mine as I have been dealing with some significant issues. The other issue is what has your diet been like for the most part, ie. highly processed or mostly lving food? That will also contribute or detract from your nutrient stores.
Low sodium, called hyponatremia (sp?) can also be a problem.
If you want to design a pre- fast plan that is nutrient, and carry it out for say a month in advance of the fast, that will carry your nutrient stores further.
Eventhough I had severe hypokalemia at the end of my fast, I still believe and have confirmed through my own experience that fasting can reverse disease and or decrease symptoms, reverse years, detox you and possibly save you from the complications of the SAD ( standard american diet) and problems as related to use of prescription medication.
I am currently on day 19 of my second water fast. It has been different from the first, but detox much less.
Lesson two : IMHO, Keep an eye on your electrolytes
Lesson three: Deal with qualified natural hygiene practitioner.
Lesson four: Feed your mind. Alot of good books out there, but Shelton and Dr. Fuhrman.com, and TrueNorth with Alan Goldhammer.
See the posts of chrisb1 and Fonty- They have been a great help and many of commentaries have websites attached to them where you can find practitioners or further info on fasting.