With a desktop computer costing from $500.00 I would hope I could get at least 10 years of use out of it before it actually dies and I have to replace it.
You can always update the ram and change the hard drive etc without having to spend as much on a new computer over those years.
10 years is about right and when I'd say I got my money's worth out of the desktop computer.
laptop computers for me tend to last 3 years but laptops are usually used more harshly and the moving them around so often tends to shorten their lifespan.