Fasts differ so much in their rules that treating them as one practice can mislead. What varies is the substance, the duration, the intensity and who is excused.
Ramadan is a daytime fast: no food, no drink of any kind including water, no smoking and no sexual activity from first light to sunset, for a lunar month, broken each evening and resumed before dawn. Because the Islamic calendar is lunar, Ramadan moves through the seasons, so a summer Ramadan at high latitude can mean nineteen hours without water. The exempt are named rather than left to conscience: the ill, the travelling, the pregnant, the nursing, the menstruating, the elderly and the young, with make-up days later or a payment for the poor instead.
Yom Kippur is a full twenty-five hours without food or water, and adds prohibitions on washing, anointing, leather shoes and sex. Judaism keeps several other fast days, most of them from dawn to nightfall only.
Christian fasting has become the loosest of the major forms. The Lenten discipline of the Latin church now amounts to abstaining from meat on Fridays and two days of reduced eating, a very considerable relaxation from the medieval rule. Orthodox fasting is far more demanding and is mainly a matter of exclusion rather than quantity: no meat, dairy, eggs, fish, oil or wine on the strictest days, across something approaching half the year.
Indian traditions more often restrict category than amount: a vow to take one meal only, or fruit only, or nothing grown underground, kept on named days of the lunar month. The Buddhist monastic rule against eating after midday is a permanent low-grade fast rather than a seasonal one, and it changes the shape of a life more than an annual month does.