There are many different protocols for fasting. I practice daily fasting with a minimum goal of 12-14 hours including sleep time. Some days I am not as hungry and can go longer. What I eat and drink the day and night before impacts my hunger the next day. If I eat low carb and don't drink alcohol or eat sugar, the fast is very easy for me the next day. On those days I often tend to fast longer.
When you first start fasting you are not able to fast for very long. It takes time to work up to fasting for longer periods. They refer to fasting as a muscle that takes time to get stronger. People who don't fast are sugar burners. Their body is relying on food coming in to use for energy. These bodies are not "fat adapted" so they are not relying on a person's own fat for an energy source. It takes about 4-6 weeks to become fat adapted.