a friend of mine had one that was about 150 gallons and it didn't look all that good it was more of a test. but it held water for 20 years. the thing is he didn't coat the out side of the wood with anything so as time past the top layers of woo were slowly pealing away. he did build a few smaller ones and I think he used an oil base house paint for the out side of the tank and those seemed to look like they were just built but were about 15 years old.. he was building a really big one, but I never saw it because i moved, I don't know if the guy is even alive anymore, he was getting up there in age..
but built right they will last a long time..