They'll last for a couple of years in a marine setup. In freshwater, they'll break down more quickly. The jaws, of course, are not bone but cartilage that will soften up fairly quickly in a tropical temp tank.
You can seal the jaws with epoxy first. Do a couple of initial coatings with extra-thinned-out epoxy. This will allow the epoxy to soak into the cartilage and add weight/strength. After the entire piece is sealed, it'll last indefinately in an aquarium.