Other guys may say I'm wrong, but from what I understand they are like anything else and do fatigue over time. If an engine I rebuilt had sat I would replace them for sure, imagine the stress on 1 valve spring compressed for like 20 years! That's one reason I try to run all my engines a few times/year. If the engine I rebuilt was running, it would depend on how well it ran and the mileage. But last forever? I wouldn't think so. Just my