I’m about 30 hours in since I last had food and I want to challenge myself to see how far I can take it. But I don’t want to push myself to the point where I could cause permanent damage to myself. I’ve read a bit about the 48 hour fast but I can’t find information on a 72 hour fast.
You body depletes electrolytes at about 48 hours so make sure you are staying hydrated and supplementing the key salts: sodium, magnesium, potassium to stay in balance. You’ll also want to be careful with your refeed; be gentle. You usually refeed over half the time of your fast. I usually do bone broth and a salted avocado with olive oil, then a light cucumber/tomato salad a bit later, then a light fish.
You can try r/fasting for more details and advice.
Are you aiming to lose weight?
If you have a solid body fat % (and are focusing on hydration and ensuring you’ve got your electrolytes), you should be fine. The record for fasting is over 300 days and tons of people on r/fasting routinely do 6-7 day fasts.
If you already have very low body fat, my answer would be different.