The answer depends on several factors. First is the choice of anesthetic or sedative agent. Some medicines last longer than others and are chosen to be appropriate for the procedure. Second is the dose of the anesthetic used. Higher doses will last longer, but may be necessary to assure a successful procedure. Some medications can be "reversed" by other medications if necessary.The goal is to prevent anxiety and allow the child to remain still during the procedure. Otherwise, the scans are blurry and unusable. All of this should be explained to you and your questions should be answered before the procedures. Most facilities provide written instructions for reference. Most current anesthetic agents and techniques are selected to allow for a rapid recovery after the procedure. Some locations use older protocols which do not involve utilizing specialized anesthesia providers and use older medications, which may result in longer recovery times afterwards. Another factor is the general health of your child. Some medical conditions and concurrent medications may slow the elimination of the sedative medications, which are typically eliminated by the liver and kidneys in the case of intravenous medications, or simply exhaled in the case of inhaled anesthetic agents. For most adult and pediatric patients, we often use the 24-hour rule, which states that although many agents are out of their system much faster (2-4 hours or 6-8 hours for most medications) than 24 hours, resuming "normal" activity is a matter of concern until a day has passed. If your child becomes more sedated or does not appear to be progressing, you may need to contact your care team or initiate a medical emergency if conditions seem to warrant that (examples would be respiratory difficulty or increasing lethargy).