Flying takes a toll on the body. You are subjected to radiation, recycled air, and varying forms of air pressure. Pilots and aircrew should take regular aviation medicals, and many countries and companies will insist upon a medical, especially for the flight captain.
It will take about an hour, but you should schedule a space in your time of at least three hours to be on the safe side. If you feel ill before going, then reschedule for another time. It is very similar to the standard executive and health insurance medicals that you have to take all the time.