While crowns are meant to last a very long time, this all depends on the way you treat the crown and use your teeth. For example, someone who grinds their teeth can expect to replace the crown due to breakdown at some point in the future. Many older crowns were "porcelain-fused-to-metal" crowns or simply referred to as a "PFM". These crowns have a metal substructure with porcelain on top to appear like a natural tooth. Over time the porcelain can weaken and chip away exposing the metal underneath. If you have a crown that has broken down in some way its a simple procedure to remove the crown and replace it with a new crown made of the most modern materials.