Catalina, Ribbon shelf live depends on the ribbon material, how heavily it was inked, and if in storage it was not exposed to air. The life really varies on a typewriter, unless it was refreshed with WD-40, the only thing WD-40 is good for with typewriters. I have some ribbons I refreshed 10 years ago and they are out on typewrites and still work fine. Good cotton ribbons from Tony Casillo las several years on a machine also. The thin nylon ones sold on Ebay and many other places dry out.
Kent, the real test is when you start eating trees like Euell Gibbons.