I have been told that the valves stem (the only part that should ever have any form of wear), should last the life of a viking or excalibur. At one time I know AKA would replace for free any of their tornado valve's if they ever went bad. Not shure what their current policy is though. If it isn't having issues, I wouldn't worry. IMO The tornado is one of the best designed valves out there.
Don't forget, the teabag mod also increases your efficiency, ROF, and helps you loose weight. Not shure If Dan Voils is still offering that service.