It depends on why you want to fast, I think.
I fast every year, usually more than once, but they are different types for the different seasons. One's coming up really soon, now, as a matter of fact.
I like to observe the Rogation Days, which happen around the turn of every season. They are a Wednesday, Thursday, and Saturday in the week of the season's start. Since they are so common, I don't do a "big" fast, just a little one in which I spend my eating time in prayer during the day except for Saturday, when I don't eat for the whole 24-hour period. Evenings are free, mostly, but I keep away from any sugared items at all. There's no reason for that last bit, it's just my own decision.
But once a year I take four or five days and go on an extended fast. For the first three days it's nothing by mouth, not even water. Then for he last day or two I drink water--the purpose is fasting and meditation, not self-sacrifice. And I pray and read the Bible a lot.
The first day or two seems to be some kind of recovery time for me, for I get really sleepy whenever I sit. So I just let it happen as it happens. If I sleep, I sleep. But then I recover mental and spiritual energy and spend a long time in prayer, reading, and meditation. Adn in learning how to meditate better. The mind wanders without discipline. The body goes into ketosis (fat metabolism that creates ketones as a by-product) during this time--my mouth tastes really foul along about the third day, so I brush my teeth more than twice a day for the last two days.
But that's part of the gain I receive from fasting. Discipline is so hard to achieve that it takes some really intense exercizing to get it. I learn more about God, I see things in the Bible that I didn't see before, I learn a lot about how messy my life is without the discipline that God wants me to have in it, and I learn that I am a pretty weak fellow, slow and dull too often, missing too many of the beautiful things that God has created in his world--including in his people.
Probably that's the greatest gift that I get from fasting. I learn a lot about myself, my demands, and what I usually think are my needs but are just really strong, selfish wants.
Fasting has helped me to become more patient with people (not "patient," but "more patient." I still have a long way to go), because I learn that I am full of the same noises and incessant desires that everybody else is. Break my routine of eating, and I become very self-centered. Fasting helps me to break that hold of my self over myself.
And like everything else that has to do with God, he always surprises me by the things he gives when we set out to oney him. They're always better than what I thought I wanted.