kanewolf :
Cases last until you get tired of looking at them usually. Mechanical hard disks have a limited lifespan. But that lifespan varies depending on how much they are powered on (total hours) and how often they are powered on/off. 5 years is a conservative lifespan for a hard drive. Motherboards are tied to generations of CPUs usually. They last until the performance of the CPU is too low and a better replacement is not available. Again 5 years is conservative. Gamers go through upgrades of CPU much more frequently.
What you are using a PC for dictates how long it is useful. Meemaw doing web searches for quilting patterns will have a much longer usability of a PC than a gamer.
Gaming. I wouldn't care much if I didn't want to play games. Last until you get tired of them? The same thing apply to montiors right? You don't necessarily have to get a new one (unless your case don't fit with the parts you want, and you want a monitor with different size and resolution).
But what about GPUs? Does a GPUs lifespan depend on what settings and resolution you are playing them on?