Gosh, Mark, I sure don't know the lifespan of an expansion tank, I would guess it would be as unpredictable as the life of a water heater. I hope you informed the customer that her TP would likely be opening whenever the tank heated. The good thing is, when she calls crying about that, you will already know what has to be done. Just take her a new tank, remove the plug, and install the new one. You make a good point about professional installers at least giving instructions about check them. I do fear however, it would have about the same results as telling them to flush the tank annually.