I do a 24-hour dry fast at least once a month, sometimes more often, just to give my digestive system a break. I've done a one-week water fast several times & a two-week fast with water & broth. I would be careful about going over a week by yourself, without medical supervision. Specifically for weight-loss, I'd recommend using
macros.
The rocky portion usually comes around day 3 to 4, which is typically when your body finishes emptying out your GI tract. Think of your gut as 3 buckets:
1. Stomach
2. Small intestine
3. Large intestine
To over-simplify, your stomach is like a blender, which purees food up & funnels it into the small intestine. The small intestine pulls out the energy (macro & micro-nutrients), and then the waste goes into the large intestine, where it comes out as a stool.
As a simple visualization, imagine your GI tract as a big, long hose, and between eating periods, you get air bubbles. When those air bubbles bulge, you get stomach pain from trapped gas. When you don't eat for a few days & your stomach clears out, that's when it will hurt the most, because now you've got a couple big air bubbles pushing on your gut until all the waste is emptied out. Then it's just hunger, which you combat by drinking water.
So the first few days are easy, then a hard day or two, then it's easy. You've most likely done this before at some point in your life, like when you've been really sick & couldn't hold anything down for a few days, then your gut heals itself & you magically feel a lot better. You can also start to understand why gut health is so important if you've ever thrown up & felt IMMEDIATELY better after doing so.
If you want to try it, I would suggest starting out a bit smaller, with a 3-day water-only fast. So for 72 hours, only drink water. Again, consult your doctor, blah blah blah. Once you learn how fasting works & how your body responds etc., it's a lot easier to manage, because you know what to expect
As far as eating styles go, I'm not a fan of intermittent fasting; I like IIFYM, which is schedule-flexible. I did OMAD (one meal a day) for about a year back in college, just because I was working a couple jobs & also going to school, so I was just super super busy & stuff like Soylent wasn't available. It worked just fine. I currently do 7 meals & snacks - I'm not super hungry in the morning, so I have a small breakfast snack, then breakfast, lunch, and dinner (not large portions), then a brunch snack & afternoon snack, plus dessert.
Sounds like a lot, but it keeps my energy up & my GI tract functioning well. An easy way to diagnose your gut health, aside from if you have stomach pain or low-energy, is how your stools are. Not to get too gross, but if they're solid, come out quickly, only need one wipe, sink to the bottom of the bowl, and don't leave a mark on the bowl, and happen on a regular basis, then your gut is probably functioning really well. If you have mush or rabbit pellets, then your diet, exercise, sleep, and stress-management typically need some adjustments.
Also, if you haven't seen it yet, watch "The Science of Fasting". It's about an hour long & is available on Youtube & Amazon Prime: