[emc7]

Glass, slate, and metal all last a very long time. The sealant will give out first, then the plastic trim. Fortunately, tanks can be resealed and replacement trim is available. In newer tanks that have a single piece of plastic for the entire top trim, the trim helps hold the tank together, so if you get a broken center brace or a split in the trim, its time to replace the trim before the tank starts leaking. Modern sealants last 10 years easy if the tank stays filled. If the tank is dry for a long time, the lifespan of the sealant is shortened. Any black sealant (came before the clear stuff) is likely 20 years old and should be replaced. Any tank than leaks should be entirely resealed as well as any used tank of unknown age.