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.
Not that we haven't tried in years past... but when you get 5-10 laptops dropping dead every month that you push past the warranty, it becomes really obvious really quickly how worth while it is to replace the things before they die. A 3 year life span is an entirely reasonable expectation for a laptop to last. Failure rates under 3 years are low, and laptops tend to stay relevant for that period of time. For gaming hardware, 3 years is a good run for staying relevant.