It is unreasonable for dentists to have a five year warranty on full coverage crowns. Many factors determine the success of a crown as many people have already listed on this forum.  It was commented earlier on this forum that the number one reason why crowns fail is due to decay around crown margins.  Decay is a multifactorial issue and certain patients are at higher risk for decay than other patients.  This pattern of decay should be discussed during the informed consent of having a full coverage restoration placed.  If the patient is informed that they are at higher risk then the situation is analagous to placing a warranty on automotive brake pads for the driver that has his/her foot on the gas and brake pedal at the same time while driving down the highway.