 This is usually conducted in the hospital, and the duration depends on the age of the person and response to the fast.A healthy adult can usually maintain a glucose level above 50 mg/dL (2.8 mM) for 72 hours, a child for 36 hours, and an infant for 24 hours.The purpose of the fast is to determine whether the person can maintain his or her blood glucose as long as normal, and can respond to fasting with the appropriate metabolic changes.