If you really want to lose weight with a fast, you need to know the science behind it. It starts with something called ketosis. Ketosis is a perfectly natural thing that happens to your body once certain nutrients dip too low. Anyone who fasts for long will experience it, as will many diabetics. Now, here's the important part so many detractors and proponents alike will skip around: before your body goes into ketosis, you're not burning fat.
Prior to this process, your body is only using up water weight. This weight will be put back on very rapidly once you begin eating again. Learn to spot the signs of ketosis, such as altered body aroma, so that you know when you're starting to actual lose the fat. Once you're in ketosis, how long you choose to continue fasting should depend, not just on the weight you want to lose, but on general mental and physical health. You may be able to fast for a surprising amount of time, even weeks on end, before needing to stop. However, others will only be able to keep it up for a few days. But regardless, if you're stopping before you're going into ketosis, you might as well not even bother. This is the reason why so many people complain that fasting-based weight loss is ineffective.
There are a number of reasons why you might not want to try fasting in the first place, of course. Do you experience a psychologically or stressful environment? Are you unwilling to take a break from your workout regimen? Do you take any form of medication regularly, even something mild and over the counter? Do you suffer any medical conditions, particularly those that mitigate the effectiveness of nutrition intake? Don't take the risk, if any of these apply to you. Instead, try a normal diet. Fasting will be there for you when you're ready for it, so there's no need whatsoever to rush.