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.
So how long is a piece of string - interesting question and this laptop question is exactly the same. A laptop can last any amount of time, and so can a desktop PC. I know you said you've had your previous desktop 5 years but that desktop could have lasted any time too.