[Fyron]

If you leave it on all the time, it will not shorten the actual lifespan of the hardware. The parts will tend to break down after the same amount of hours of operation either way. It will shorten the perceived lifespan in terms of absolute years in that fewer measured days will pass before parts tend to break. However, by the time it starts to die in either case, it will be highly obselete and in need of replacement anyways. In either case, your computer parts will probably last for many years, assuming you do not abuse them. Keep them cool, don't overclock them, etc.