How long do dental build ups last?

It really depends upon how big the build-up is and how much natural tooth is
left. Often times, we will “build up” the tooth to remove any decay or
cavity present and also since we “bond” the build-up in place, the build-up can
actually strengthen the tooth to keep what is left of the natural tooth from
breaking. My recommendation would be to have the build-up done to create
a solid foundation now and then as soon as possible have the crown done.
The longer you wait, the greater chance that the tooth could break.