"fasting" 20 out of 24 hours makes a lot of sense to me. (currently--but that tends to make less sense in the cold of winter.)But this way of living is only something I'd ever advise or recommend for someone who's already radically changed their relationship to food, and who gravitates mainly to high-vibrational living fruits and veg. And, whose mineral reserves are good, *which usually involves a period of concerted replenishing),and who's perhaps completed one or more longish fasts--over two weeks, but not more than four. The latter, in my experience, are the thing that really changes the way the body utilizes and seeks its nourishment.
Otherwise, one is often skirting the edge of malnourishment.
But as to shorter fasts, while a person is following a path of clearing a lifetime of toxemia, and is re-setting the appetites, and so on, for many people it works to build up from three days to a week, to say, fourteen days.
It's about gaining one's own confidence, and a sense of certainty--this is too often undervalued as a major aspect of a "good" water fast.
The old fasting masters always advised, do the least you can do, to accomplish the most. I take that to mean, be sensible, wise, and don't approach Water Fasting as a way to compensate for poor eating habits--which might tend to become fashionable. . .