This varies considerably. Are you overweight? Are you skinny? An overweight person may be able to fast for months while an underweight person may only need a few days at a time. I’d suggest startingslow, obviously with only water, and push yourself as far as you can before your body starts telling you its hungry again. Listen to your body, there isn’t a one size fits all here. For myself, I believe 4-12 days to be my comfortable range. I’m pretty slim already, and I want to try to push for more days, but I look scary. 4 days is said to be a good point where you get a lot of the benefits of fasting. Autophagy, hgh, immune system reset, digestive issues start to resolve etc. 21+ days and people report healing their long term tinnitus and hearing damage, which I want to go for.
Read up on dr longo and dr fung. They can give you every bit of info. Dr Fung book is especially helpful, but fasting is as simple as it sounds. Stop eating until you are hungry. Drink water. Feel dizzy? Take some salt.