I'vehad my root canal several years ago and still hadn't put the crown over it. if i do that, will the tooth last a life time?

Possibly.: The am assoc of endodontists recommends that all posterior teeth that have root canals be protected w onlays or crowns.. This protects the tooth from fracture. Before crowning, i also recommend a bonded core be placed over the canals to prevent leakage and strengthen the tooth. When all is done properly, you can expect many years of use. Teeth can still get decayed and/or ger periodontal disease.

Only one certainty: No one can guarantee a lifetime of anything, but the tooth stands a better chance with the crown than without. It is also difficult to measure how much damage the tooth has undergone without a crown in place for the last few years. Have your dentist evaluate the tooth in its current condition for the best course of action.