Hi,
Then Esther bade them return Mordecai this answer,
Go, gather together all the Jews that are present in Shushan, and fast ye for me, and neither eat nor drink three days, night or day: I also and my maidens will fast likewise; and so will I go in unto the king, which is not according to the law: and if I perish, I perish. <-----> Esther 4:15+16
Did one of these years ago, and it gave some distinct comfort in knowing by experience that life could be sustained even in the absence of food and water...but have experienced great healing and deliverance from longer fasts which were water based (14-30 days).
Fasting without water is much more taxing to the body, and to avoid being overtaxed, 3 days is plenty of time to gain the experience, and one should be very still and quiet during that time, avoiding labor, exertion, exercise, anything which would break a sweat.
With neither food nor water an average person might live 7-10 days before expiring. But with water yet no food, people have gone for 50-60 days before dying.
Hence, 3 days is a good max-out point for no food or water, and 40 days a good maximum for water included fasts.
As for "juice fasts", one could probably live many, many years with solely ingesting various concoctions from their blender/juicer; as a full line of nutrition can be gotten that way.
And apparently, the way most Muslims fast, they only hold off eating and drinking from sunrise until sundown, and then eat and drink to their full. (Which prevents the experience of a "long term fast", whether it be 3 full 24 hour days without food and water, or several weeks without food.)
wm