Never really went longer then 5 days, but I heard stories of people going complete 30 days without food consuming only water with some minerals and salt.
From Wikipedia:
Angus Barbieri's fast. Starting in June 1965, Scottish man Angus Barbieri (1939 – 7 September 1990) fasted for 382 days. He lived on tea, coffee, soda water and vitamins, living at home in Tayport, Scotland, and frequently visiting Maryfield Hospital for medical evaluation.
But AFAIK the Guinness book no longer collects fasting records, because they do not want to promote anorexia.
I've water-only fasted a few times, working my way up from a few days to about thirty-one days twice (with the long fasts a few years apart, most recently last year). I would have gone longer except for work and family obligations.
Related recent books include "The Pleasure Trap: Mastering the Hidden Force that Undermines Health & Happiness" by Douglas J. Lisle, Ph.D and Alan Goldhamer, D.C. (of the TrueNorth Health Center) and "Fasting and Eating for Health: A Medical Doctor's Program for Conquering Disease" by Joel Fuhrman, MD. There are much older books on this though, including by Herbert Shelton.
Fasting may help improve memory function by stimulating the production of a brain-derived neurotrophic factor (BDNF) which helps with learning, memory, and making new nerve cells in the hippocampus. Fasting also helps the body clean up dysfunctional cells (including potentially cancerous ones).
So, there is probably a good health reason why every major religion has fasting as part of its rituals. Also, it makes a lot of sense why agrarian civilizations might fast in late winter or early spring when food was running short, but to suggest pregnant women and young children should be exempt (so, making a virtue out of a necessity). For example, Christian Lent in 2020 went from February 26th to April 9, which would historically have been a time of low food supplies in the Northern hemisphere.
While not exactly hibernating, I definitely had lower physical energy. So, walking the dog a mile or two each day or two was harder. In general, I've seen knowledgeable recommendations not to do much exercise at all when fasting, although others disagree to some extent.
For people who want to do a medically-supervised fast, consider the TrueNorth Health Center in Santa Rosa, CA (although I did not go there myself) who wrote the first book I referenced. https://www.healthpromoting.com/
I feel the USA would be a much healthier place (with much lower health care costs) if there were such fasting-oriented/nutritionally-oriented health centers available in every community -- with a month or two spent there covered by health insurance and some sort of new medical leave act. It's ironic that most US health insurance will pay $100,000 for weeks of hospitalization and other treatments related to a heart bypass operation (which does nothing about all the other clogged arteries in the body) -- but "health" insurance generally won't pay $10,000 (or whatever) for someone to take a few weeks away from work (the biggest part of the cost) and go to a place like TrueNorth and begin the process of unclogging all their arteries (including in their brain). I have no connection to TrueNorth; I'm just a fan and believe their book helped me on a path towards better health. Perhaps there might be a YC investment idea in there for somebody?
There are also several YouTube videos of people documenting their long water-only fasts done in various locations.
In general, you can probably get at least some of the benefits by just shortening your eating window during the day to six to eight hours or less (e.g. only eat between 10am and 6pm or such). That gives your pancreas and cells a chance to rest and recover from producing and using insulin. That's one form of "Intermittent Fasting" most people could do every day without impacting their social life or work life very much -- just skip either breakfast or dinner and don't do late night snacking.
Vegetable juice fasting is also another thing consider, like in the inspirational and funny movie by Joe Cross called "Fat, Sick, and Nearly Dead" and its sequel. But such juice-fasting is not much of a true fast. It mainly just supercharges your nutrition (which is a good thing).
One of the most important things about a long fast is to break it gently. People disagree slightly on exactly what light foods might work best. Some suggest an orange; others suggest orange juice or vegetable juice. I've tried all of those with good results. But everyone agrees definitely no cheeseburgers or pizza or whatever other heavy food to break a fast -- which might cause serious harm or worse. It's kind of like how landing a plane (breaking a fast) is a more dangerous time than flying one in the air (fasting).
Since your body will detox some going into a fast, it is best to eat a clean diet (e.g. organic, whole foods) for months or even years beforehand to make the fasting process easier without many detox headaches or such. At the very least, ideally eat cleanly for a few weeks before any water-only fast and/or do short vegetable-juice fasts first to get a sense of how your body responds as it slowly detoxifies.
I also feel like from shorter initial fasts my body sort-of learned how to activate the fasting machinery and put into use alternate biochemical pathways -- and so future fasts were much easier to get into.
While fasting can be healing, what may well be more important long-term is to eat a clean whole foods diet as much as possible your entire life. And as above, the two complement each other -- otherwise fasting will be a much worse experience as your body tries to detoxify a lifetime of nutritional mistakes and environmental toxins sequestered in body fat at once. Millennia ago, with most people eating simply and living in places without heavily-polluted air and water (Ancient Rome excepted, perhaps), fasting would generally probably have been much easier in terms of few detoxification headaches compared to someone these days who has been eating the Standard American Diet and lots of junk food and living in a city with lots of air pollution, lead contamination leftover as dust from leaded gas, PCBs, mercury from coal burning getting into seafood, and so on.
It's a shame that the USA spends over two trillion dollars a year on "health" care with such poor results -- including spending for endless expensive treatments and drugs for many issues that fasting would help with at lower cost. Plus, for all that money spent every year, there is little research into things like fasting (although there is some, like by Mark Mattson: https://en.wikipedia.org/wiki/Mark_Mattson ). Of course, there is not much money to be made recommending people just stop eating for a while...
It's been joked (for thousands of years!) that the first half of what you eat feeds you, the second half of what you eat feeds your physician.
Ironically, after people get one of the diseases fasting can help prevent like heart disease or cancer or diabetes, often people have to fast for a time before various surgeries and then change their diets afterwards. If only they would have tried that beforehand.
In any case, especially having been raised by a mother who was traumatized by the Dutch Hunger Winter during WWII, it is good to know I could go without food for a couple of months if I had to -- and to know that my health would be better for it, even if my work might suffer some in the short-term (but gain in the long-term). Still, it is a different thing to have lack of food forced on you and your family and community as a result of a war versus choosing not to eat for a time for health reasons in a land of abundant food supplies.