You have no way of knowing how long your body can fast. Take it one day at a time and if you really can't keep going because of potential dangers to your health, stop the fast.
You can always do another one.
I wanted to fast for forty days, but on the night of day 31, I stated to experience irregular heartbeat, muscle cramps, and dizziness and weakness. So I broke the fast early the next day. I probably could have kept going for another 8 days, but since I was fasting without supervision, I decided to err on the side of safety and stop.