My experience would tend to suggest that 2-3 years would be typical for a PC.
Even if the machine is still working at the end of 3 years, the software and OS upgrades? will have slowed it down to a crawl.
In a networked environment, you could consider VDI or Terminal Services/Citrix etc and using thin clients. The thin clients will outlast the PCs and your hardware refresh will be limited to the server.
Some of our thin clients use as little as 7 watts, so you could be helping your fuel bills and the environment too.
I am on my 5th desktop computer since 2000 (Y2K).
The most common thing that seemed to give out first (within 3-5 years) was the Power Supply, which I successfully replaced each time - with the recommended replacement.
However, it seemed like some (unidentified) components of the Motherboard(s) also failed on each and every one of my previous desktop computers.
Not knowing enough about computer hardware, I took a couple of my first ones to some folks that were in the business of repairing computers for a diagnosis and possible repair. (this is how I found out about the Power Supply?s)
After a few rounds of this, when the computer that I had broke down, I decided to just get a new one (in the $500 - $1000 range) and donate the broken one to a non-profit computer repairman that gave his repaired electronics to charities.
I have had as many as 3 desktops at one time, but never one for more than 5 years - even though I work to keep them in good condition (like vacuuming them inside and out ever 3 months, etc.)
I do realize that since BIC invented disposable lighters that it makes more sense to buy new rather than purchase fluid, wicks & flints. And - these days it costs less to buy a new digital camera than to get one repaired - But, is it Now also the same with the average desktop (generally better to buy new rather than repair)?
Whether or not that's the way it is these days, does anyone know - What the Average life is of an Average Desktop Computer? (Average = common brand name, $500- $800)