I have done a few fasts from 3-30 days for health reasons. I have a lot of joint pain from leaky gut and when it gets really bad I juice fast because most of the pain goes away during the fast. I call them “juice feasts” because I juice mostly green veggies with apples and drink as much as I want throughout the day. I end up drinking at least a gallon a day plus water. The 30 day fast was in 2014. I lost around 25 pounds and continued to lose a little weight after that year. To prepare for the fast I made sure I had a good juicer and talked to a nutritionalist who specialized in fasting.
In 2018 I changed my diet to WFPB (mostly veggie based foods). I ate unlimited amounts of (very healthy and clean) food again, lost 60 pounds and got down to my goal weight.
I am a stress eater and this past year I’ve let a lot of things get to me. I gained back about 15 pounds and just did a 5 day fast, ending today. I just needed a break from the crappy processed food I was eating to reboot my body and get the sugar out of my system. I am planning to start IF in 2023 along with WFPB eating to keep my joint inflammation and massive headaches to a minimum. I’m planning on doing 16/8 and eating between 9 and 5.
With all of that said, I think the length of your fast depends on the reason you want to fast. I did it purely for health reasons and my first thought is that if you are trying to lose weight, IF seems to really work for loads of people. I’m run and bike at least 4-5 days a week so I’m hoping that between exercise, IF and not eating crappy processed foods I can drop about 10 pounds which will make my joints happy.