What is the warranty period on these crowns?

Some states make "warrantees" illegal, so you won't find anything official or legally binding.  However, good sound business principles are in play, so many dentists will redo work within the first year or two, but not because they "have to."  Its just good business.Dental insurance usually will pay for a crown on the same tooth after 5 years, but some plans are changing to 7.

Most dental work does
not carry a warranty, as abuse or neglect can cause restorations to fail before
they ought to. That being said, the average life span of a crown is
approximately 10 years.

There is no formal warranty period on any dentistry.  As a matter of fact, the dental practice act in many states expressly prohibit "guarantees" and "warranties" by dentists on their dentistry.However, that being said, many dentists will replace broken crowns or veneers if they were recently done.  After a few years that diminishes.  And some dentists will do replacements, but require the patient to at least pay a portion of the bill depending on how many years ago the broken veneer or crown was originally done.Although most dentists strive for excellence and would like their dentistry to last a  lifetime, that is not always achievable for a number of reasons beyond the control of the dentist.You might ask yourself how long your car was guaranteed without buying an additional service contract or extended warranty?  Car problems a few years out can occur for a myriad of reasons, and likewise there can be a myriad of reasons why dentistry that fails or breaks.