I have done several water fasts, some shorter and some longer. The longest was 35 days. I never actually fainted, but towards the end of that 35 days fast, there were a couple of times when I had to sit down fast, because I felt like I was going to faint. However, by that time I was already seriously underweight (body mass index of around 15). This fast was mainly for cleansing reasons and that's why I continued for so long. If you are fasting for weight loss, then you will probably not continue until you are at such a low weight. Of course, if you have more weight to lose, then it will take more than 35 days to arrive at that stage.
Just be careful not to stand up too fast from a sitting or a lying position, as when you are Water Fasting
your blood pressure is lower than usual. Getting up quickly will probably not really make you faint, but dizzy nevertheless. Good luck!