The length of a fast is determined by ones food reserves, and I do not mean how much fat one has.
Food reserves also mean reserves of vitamins and minerals and other nutrients such as phytonutrients as well, so if a pre-fast diet is largely plant-based: fruits, vegetables, nuts, seeds, legumes and pulses, then food reserves should be more than adequate for even the most protracted of fasts.
The absolute limit to the length of a fast is when the return of hunger has been reached. This could be anywhere from 30 to 50 days for most people of normal to above average weight, and only if water is consumed according to thirst.
Food reserves are preserved by resting as much as possible with freedom from stress and anxieties: this will shorten the length of a fast by wasting these reserves which are consumed much more rapidly this way.
The return of hunger is an unmistakable signal (intense hunger which has to be controlled for fear of overeating) from the body that it is about to enter the starvation phase and organ damage occurs......................
"There is a tendency on the part of the faster to overeat, not alone because he is hungry, but also because he is desirous of regaining his weight. His friends also urge him to eat. Upton Sinclair truly says: "A person at the end of a (long) fast is an agitating sight to his neighbors, and their one impulse is to get a 'square meal' into him as quickly as possible."
"In general I agree with Carrington that "natural hunger, and that alone should indicate the terminus of the fast; when the fast is ready to be broken. * * * The artificial breaking of the fast; the taking of food in the absence of real hunger, for the reason that the ignorant attendant thinks the patient has 'fasted long enough,' is an abomination, and an outrage upon the system which cannot be too strongly deprecated." Most fasts are broken too soon; that is, before the work of renovation is completed.
The care that must be exercised in breaking a fast is in proportion to the length of the fast and to the general condition of the fasting individual. The approved plan is to break the fast on liquid food, using for this purpose fruit juice, or tomato juice, or Watermelon juice, or vegetable broths. Fruit juice--usually orange juice--is used most often.
Orange juice, grapefruit juice, or fresh tomato juice are excellent with which to break a fast. Watermelon juice or the juice of the fresh pineapple or of fresh grapes may also be used. A half a glass may be given at the start. After an hour, another half glass may be given. Juice may be given every hour the first day. The second day a whole glass of juice every two hours may be employed. On the third and fourth days give the whole orange or grapefruit and on the fifth day other foods may be added. Large meals should not be attempted in less than a week. These instructions are for the long fast. A short fast requires less care in breaking and is usually followed for several days by an eliminating diet"........................... http://www.soilandhealth.org/02/0201hyglibcat/020127shelton.III/020127.ch30.htm
I envy your excursion to the desert for that length of time. An absolutely ideal way of fasting in the peace and serenity of Nature. This was also Paul Braggs ideal way of fasting in the solitude of the desert as well.
Given sufficient time, your lengthy fast will bring you much closer to God: a spiritual/reality experience that is just too profound and great for words to express.