They are long-term restorations
Crowns and bridges are designed to last for many years, but their lifespan depends on the tooth, the material used, the bite, the gums and how well they are maintained. As crowns are more simple than bridges (being effectively one independent part of a bridge), they tend to last longer than bridges alone. We are aiming for 20+ years of longevity, but whether we hit that mark or not depends on how well your take care of your work after it's placed.
The tooth underneath still matters
A crown or bridge protects and restores the tooth, but the tooth underneath still needs to stay healthy. Decay can still develop around the edges if plaque is left behind, especially near the gumline. The crown / bridge can't decay, but the tooth beneath it still can.
Bite forces can affect longevity
Clenching, grinding, an uneven bite, missing teeth, gum disease, inadequate hygiene and decay can all place extra stress on crowns and bridges. These factors can increase the risk of chipping, loosening, decay or failure over time.
Maintenance helps protect the result
Regular dental check-ups, good brushing, cleaning between the teeth and a night guard where appropriate can all help crowns and bridges last longer.