How Long Should a Desktop Computer Last?
A well-maintained desktop can reasonably last six to eight years or more, since components aren't as tightly packed or heat-stressed as in a laptop, and individual parts (drive, RAM, power supply) can usually be replaced or upgraded rather than replacing the whole computer.
Why desktops tend to last longer
Desktops generally run cooler than laptops relative to their performance, and their components are more accessible for cleaning, upgrading or individually replacing when something fails, rather than needing a whole-unit replacement.
What determines the practical limit
As with laptops, the processor and motherboard eventually become the limiting factor, since they're the least practical to upgrade. Storage, memory and the power supply can usually be addressed individually as they age. This page will be expanded with more detail in a future update.