Doctor's Dierespect of Others

When does waiting in a doctors reception room without being seen by said doctor shift from being an acceptable waitng time to being down right RUDE on the part of the doctor? On 2 occassions I have been well on time for my seperate appointments, only to be kept waiting for over 2 and a quarter hours without being seen by the doctor. Why is it that a patient has the manners to be on time for their appointment and wait patiently, but the doctor has no regard for others?