If you define life span as from the time you bought it, to the time it dies, I have not had one that died on me yet. And I have owned PC and laptop since the early 80's.
There is not much to keep it running. Blowing out the dust from the internal/fan plays a key part. Maybe picking dirts that stuck on the keyboard too. They just run. Oh, imaging it regularly to prevent virus attack is essential in these days.
Software/speed is another issue. Machines tend to get slower because of the junks we loaded into it and the bloatwares that got downloaded with or without you knowing it. Newer applications utilize higher speed hardware that older machine have a hard time to handle.
I always "trade in" my machines 5-7 years. Thanks for eBay where plenty of people are not as fortunate as me are willing to pay to take it off my hand. For that reason, I proudly pronounce that I have never created any electronic waste.
Well, cellphone is another story.