Each system is different, and each component is different. I have had PSU's go out after 6 months. I have had identical PSU's keep going. It's really hard to tell. If I were to guess, I would say that I folding rig should last at least a couple of years(if not longer). If that rig had a mild overclock and was ran 24/7.
The rig will last many years. I remember an article by intel saying that their CPUs will last 10+ years loaded 24/7. As long as its cooled well and voltages are safe it should last long enough that it is cheap to replace when its dies. I expect 5+ years.
Edit: As gibby pointed out with the PSU, if you don't have stable voltage at your house you can kill PSUs quickly. They are the only component that I have killed except for mobos and RAM while being stupid.
Lifespan of a rig depends on its maintenance. If you make sure you replace TIM every now and then, give it a good dusting, and don't fiddle around. I believe you can have a rig running as long as you want to
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