I have been keto about 3 months now.
I accidentally did a 24 hour fast my first month, but that was more because of appointments, traffic, forgetting my lunch at home and stuff. i got hungry, but it was easier than I thought, at that time I was not fat adapted, so i was at my limit.
At 6 weeks I KNEW I had hit fat adapted, its like a light was switched, more energy, no hunger, felt great. I had this exact same question you did, and I decided i would ride into the 2 month mark with a 72 hour fast. First day was a breeze, getting past the 24 hour mark was easy compared to the month before! Second day was not bad at all, but I hit hour 48 and suddenly felt horrid. Dizzy, lightheaded, and just could not focus on anything except the fact that I had not eaten. So I decided that this was my body telling me STOP NOW! So I did, broke my fast then at hour 50 and felt instantly better.
Now last weekend I tried to do 72 hours again, and I made it. Getting through the 48 hour mark was much easier than the previous month. I actually felt like most of the third day i wanted to continue it further. But I got to hour 70 and that quickly changed. Again i got lightheaded and dizzy and my legs started really cramping up and I broke the fast right at 73 hours.(With a nice steak and lobster dinner too)
So that’s the long story haha. The short of it is this. If you are fat adapted(and you will KNOW that you are) go for it! If you run into a road block don’t hesitate to break early. Fasting seems almost like a muscle, we need to flex it and build it up. So if you can only make a day, that’s fine, maybe the next time you can do two, then three, and just slowly ramp it up.
Most importantly just listen to your body, it will tell you when it is time to break a fast. And it can be sudden, like my last one I literally felt like I could go for days and days more, then suddenly right at hour 70 that all changed and I was struggling, my body needed to break it so I did.