The only perishable item is any rubber gasket. The spring is more or less at rest on the shelf. The other metal items do not perish. As you said, they are so cheap that if you have an old thermostat (NOS), it's not worth taking a chance. The expandable compound inside (like wax) could degrade I suppose. As far as how long, can't say, depends on it's storage environment and quality of any rubber. No rubber= long time.
You have brought up one of my pet peeves. Bunch of NOS stuff on Ebay and elsewhere. Many people get all tingly over NOS..... Shocks, Carb Kits, all kinds of parts that contain rubber, plastic, gaskets, seals, insulation, fluids, diaphragms, compressed or extended springs, etc. Those compounds/materials will be no good after 50 years. They were not as good as todays materials. So be advised about NOS. Make sure you won't be relying on them if they have perishable/degradable materials or are made with an item that "takes a set"/looses it's "memory. NOS=NFG.
