Basically no dental work lasts forever. The same goes for replacement joints. You kind of have to judge how long you expect to live and whether you should spring for the 25 year hip or the 15 year hip. It's pretty anxiety inducing.
If you've got fillings, crowns, root canals etc. it's important to keep getting checkups because they can see the beginnings of failure on xray and fix them before they become a real problem. Four to five years is bad, but it could be because of individual conditions. My mom had a root canal fail after 15 years.