It will come down to the quality of the components. If the components are cheaply made like Deer power supplies, or PCChips motherboards, than I can say they won't last long. If the parts are higher end from companies like Enermax, Asus, and of course Intel, there shouldn't be any worries as to how long the parts will last.
IMHO, this question is rarely asked as most advanced users will dump their hardware for the newest components possibly every 6 months to 2 years. I don't see the average computer lasting anymore than 5. If the system is a high-end server, they should last for years minus a few dead hard drive replacements or redundant power supply replacements.
I don't know if this was the answer you are looking for, the question is somewhat vague.