I have experience in the industry (from the side that designs things - worked/working for an ODM and OEM).
The bathtub curve. Oh, the bathtub curve. Basically, parts have a high infant mortality; this is why they get burned in before being shipped, so that a lot of the infant mortality happens in the factory / etc. Sometimes you will still see it, especially from cheaper vendors using cheaper parts. After this, your failure rate is very low until a certain amount of time at which point things start breaking because parts were only designed to live about that long.
For CPUs, that number is 7 years.
For other parts, that number is higher or lower.
The weakest link tends to govern how long your computer lives. The cheaper the computer, the cheaper the weakest link. It's rare for it to be a CPU these days - I don't know AMD's bathtub curve, but Intel's 7 years is pretty generous. That's 7 years running at 105C, last I checked, which was admittedly a few years ago. But capacitors tend to be weak points... caps in your power supply and the system won't boot, caps on your motherboard and the system won't POST, caps on your video card and the card goes wonky or doesn't work, blah, blah, blah. Then there are the more heavily-used components like hard drive controllers, blah blah, that are always on and unlike a CPU don't have a high dynamic range. And the RAM, oh god the RAM, a single chip failing just a little causes entire systems to not boot or be very wonky, and you have to diagnose which stick is failing and remove/replace it. Easy to do but hard to figure out if you're not aware of the symptoms.
This is a very long way of saying:
- It depends on your luck
- It depends on how much the system cost
- It depends on how good the OEM is at selecting good parts
- It depends on how well you take care of it.
I expect all my gear to live at least five years. My laptop is still alive five years later, and still a beast of a machine. My HP tower before that died after five years because HP is a piece of shit company making piece of shit gear (see point 3). On the other hand, I expect my current PC to last (with upgrades) 10 years or so. Why? Because I bought top of the line, including cooling, and don't push things to run hot 24/7.
$350 in 2010 from Dell I would expect to be barely alive in 2014. Dell's budget line is a real piece of shit, but to be fair, that's true for everyone's budget line. The problem is that the margins are just too slim, so corners are cut. I'd much rather buy a device that costs $200 to manufacture for $350, than a device that costs $325 to manufacture for $350, because while I'd get more for my dollar from the latter, the former would have much better quality control because those margins allow for good quality control. And good design and manufacture and parts selection.