One year. There is a ball in the sheep's stomach, also called cholesterol stone. The first condition of formation is the increase of cholesterol saturation in bile. The formation of stone needs a core crystal as the starting point. This core crystal can be tiny cholesterol crystals or other substances, which gradually increase in bile and attract more cholesterol crystals to deposit on it. Once the core crystal is formed, it will gradually grow in the gallbladder, so that the cholesterol crystals will gradually gather together and gradually harden to form stones. This process needs nearly a year of precipitation.