There were four basic materials that the Egyptians used to build the pyramids: limestone, pink granite, basalt, and alabaster. Each type of material had a diverse use. Limestone had, for the most part, two uses.
To answer your question: The useful lifespan of a CPU is about 3-4 years. My guess on the technical lifespan under full load is closer to 8-10 years if nothing else breaks and takes the CPU with it. The reasons: High Performance Computing Clusters usually keep the computers for 3-4 years.
Desktop computers have at least 5 different components, and the components each have their own lifespan that ranges from 10,000 to 50,000 hours or more. We consider 50,000 hours to be the end-of-life range for most computers, since it represents almost 6 years of continuous operation.