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.
The most common problem is that the motherboard's capacitors go out first leading to a non-bootable(and thus non-usable laptop). These capacitors have up to 5000 hours of life. A rough estimate for someone who uses laptops as their primary computer (e.g. without a desktop) is 5 hours a day average; this yields 2.7 years.