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.
Simply put, a clean computer can last longer. Dirt and dust buildup in and around your computer can clog the fans and air intakes, causing your hardware to run hotter, which lowers its expected life span. So if your PC is looking a little musty, take the time to clean it.