New discoveries have found exceptions to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong. Hail to the Father of Genetics ! The name is ma-ma-ma-ma-ma-Mendel.
Dental crowns should last on average from 10 to 20 years. Crowns are still subject to fracture and cavities, so it is important to take extra care in brushing and flossing around crowned teeth to prevent them from needing replacement too often.
The short answer is that properly done porcelain veneers and crowns usually last a very long time! They are like a car, how long your car lasts depends on what type of car your purchased, how well it was maintained, did you follow through and have all your routine services done, in what conditions do you drive it, how many miles do you drive every year, and did you get into any automobile accidents.