Fasting is different for every person and every fast is different for each person. With that said I recommend setting a small goal and parameters for each fast. For an example:
I will fast on water only for (3) days. I will work out each of those days if on a scale from 1-10 my energy level is 5 or above. If my energy level is below that I will soak in Epsom salt and give my body rest.
I also advise planning how you will break your fast prior to beginning it. How you break a fast is important and will either aid or hurt your body.
From the ketones that are being burned and the caffeine that I put in the water. I find that I use less caffeine while water fasting because of the ketones. I also take 1/2 tablespoon of coconut oil that is turned directly into ketones since they aren’t processed by the liver.