When I started Water Fasting in the early 1990's, I fasted anywhere from 24 to 32 hours once every week. I now fast for 24 hours once a week. I fast from around noon on Wednesday until noon on Thursday.
Short fasts are beneficial. You do detox and it gives your internal organs a rest. I use it as a way to maintain my weight and it gives my body a break from digestion. Your liver will thank you. I experienced much improved physical health from short fasts. They also helped prepare me for my longer ones.
Paul Bragg in his book "The Miracle of Fasting" states shorter fasts are better and safer. He advocated 1, 3, and 10 day fasts.
Dr. Sheldon and Dr Fuhrman were for longer fasts..10 days plus.