How long should a girl fast for?

Guidelines for Fasting

On a fasting day, it's recommended that you limit your intake to 20–25% of your normal calorie consumption, which corresponds to around 500 calories. One popular approach is the 16/8 method (often referred to as the “Leangains method”), where you fast for 16 hours each day and consume all your calories within an 8-hour period. For women, it's suggested to begin with a 14-hour fasting duration and gradually work up to 16 hours.