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.
Usage of a CPU does result in wear at the atomic/electronic level. The actual lifespan of the silicon transistors of a consumer CPU is typically in the range of 20-30 years before there is a failure, not 3-4years. It is asssumed by then that the item would be obsolete.