Based on my experiences working with computer systems that run 24/7/365, most components in computers that remain turned on that long have a life span of about 2 to 3 years. Power supplies and hard drives tend to die in about 2 to 3 years. Replacement of these two components are relatively inexpensive. In rare instances I have had to replace system boards, memory, and LCD screens.

I believe the average home user who turns off the computer every night will have a computer that will have a life expectancy of 5 to 6 years.