Our clinic has signs posted all over that if you are waiting longer than 30 min. after your appt. time, to see the receptionist and they will reschedule if necessary (at no cost). I think that is probably the best way to handle it... It's understandable if a doctor gets behind, esp. if they have to respond to an emergency issue (like missplaymate618's OBGYN) so just let the patient reschedule so that they aren't wasting their time waiting for hours to be seen.