The life span of pantyhose can vary depending on how often you wear them. Washing them--which is a necessity--can also contribute to the degradation of the hose. As long as the hose still look good and are free of runs, you can wear them for as long as you like.
I had a pair I put on just yesterday, the elastic in the waist was so old it left black rubber marks all over my hands and stomach. Luckily my dress was black too. In other words my tights were like ten plus years old, so maybe not keep them 10 plus:)