Honestly AI overviews are a pretty good guide, ask about a 3-5 day water fast. I was nervous going into my first one, but now I don't worry about it. The main thing is to drink a lot of water and have 1-3 LMNT electrolyte packs (unflavored, no sugar) per day, depending on how much you're sweating/exercising (which you absolutely can do, especially the first 2 days).
You can expect to feel colder as your body doesn't conserve as much heat, and after ~2 days more lethargic physically, but your mental energy may actually be higher. I don't sleep as well when fasting, so 3 nights is about my limit. That being said, you feel rested on less sleep, because your body is probably producing a lot less waste.