Hmmm, good question. I think it's been a good four years for sure. I've used it on a '80 F*rd Courier pickup which had constant rust problems on the fenders behind the front wheel and the bed. After a year or so of the Rustoleum, evidence of new rust would start to show. But I just took a close look at it and the paint on the Zerorust primer looks just like it did when I applied it. Very solid and no evidence of rust. I'm happy with it. Especially since I didn't do that great of a job grinding off the old rust or applying the primer.
