How Long to Fast?
First of all, to clear up any misconceptions about how long the body can handle, A.J. Carlson, Professor of Physiology, from the University of Chicago, states that a healthy, well-nourished man can live from 50 to 70 days without food, provided he is not exposed to harsh elements or emotional stress. In addition, human fat is valued at three thousand five hundred calories per pound. Each extra pound of fat will supply enough calories for one day of hard physical labor. Ten pounds of fat are equal to 35 thousand calories! Most of us have sufficient reserves, capable of sustaining us for many weeks.
1 Day, 3 Days, 7 Days, 10 Days 21 Days, 40 Days are the types that are seen throughout Scriptures. You could do a fast with just eating fruits and vegetables, or just juice & liquids, or just water. If you really wanted to do no food or liquid that is also an option. The only thing is that you SHOULD NOT go longer than 3 days, because there might be some bad outcomes, like loss of membranes in the brain or death.
How to Choose the Length of the Fast
1) If you have fasted before (Start with one day, if it's your first)
2) How many days you feel God is leading you
3) If there is any major events going on like Holidays or Parties or places that it is really rude to not eat.
4) Your work schedule: If you are working a 40 hr week of really strenuous work and really busy it might not be such a good idea to do a 21 day or more fast if you’re only drinking water. It is possible, but you won’t really have a lot of energy to perform at your best. It might be a better choice to do a juice fast, to keep your energy up.
5) The purpose of your fast could result in a certain amount of time too.
*It is a good idea to determine the length of the fast in advance, because the enemy will be on your back constantly, so you might be more likely to quit if you just leave it up to your feelings.
*It really isn’t about the length of time; it is about where your heart is. If you can only do 12 hours and your heart is in the right place than God will be pleased.