The lifespan of the average PC is almost impossible to measure. Our moderator states he has one that has been in use for the past 10 years. In our office here are some that are between 6 and 7 years old with some about 2 years old.
But the question also asked about when you could expect the components to start failing. That is a more difficult question to answer. The two things that are most likely to fail first are the CRT monitor and the hard drive. The monitor because the phosphor will become etched or the electron gun will become less able to provide enough excitation to drive the screen to its desired brightness.
The hard drive will fail because of mechanical motion, either the actuator that moves the heads across the surface of the disk or the motor that spins the disk. Those are two of the most common failures. The other common failure would be one of the electronic components such as an integrated circuit or driver transistor. These usually fail because of heat. If the case fan fails, then the operating temperature climbs and for every 10 degrees Celcius (about 18 degrees F) the junction temperature rises in a transistor, its life is cut in half. If an IC is spec'd to run say 40,000 hours at 30 degrees C (86 F), then if the temperature climbs to 40 C (104 F), the life span is now 20,000 hours.
This also assumes the IC is powered on for the full time and the operating temperature can be maintained. What is not counted is thermal shock, the almost instantaneous heating when electricity begins to flow. The buildup of dust and dirt in a computer case also adds to thermal stress as the dust and dirt inhibit the flow of cooling air and act as insulation to keep the heat in the unit. Opening the case at least yearly and running a vacuum inside is one method of reducing this problem. You may also want to use compressed air, or even the exhaust of the vacuum cleaner to blow out any dust or dirt the suction misses.
So, how long will a computer last before it fails mechanically? It depends on the environment. Used for only a couple of hours a day and properly maintained, almost indefinitely. I'd worry more about my software. But if a computer is turned on and left running behind the closed door of a cabinent and not checked and cleaned regularly, then maybe 8 or 9 years if you are fortunate.
Rodgers Platt