Replace when failure happens
I only replace the motherboard, cpu and graphic card when a failure happens. That happened for me this April when the motherboard crashed (along with the dvd drive). Normal lifetime of a PC today is around 5 - 8 years, after that the capacitor and other chips wear out and just start to fail. Hard drives are replaced when they fail and that is normally every 3 - 5 years at the moment. It is not a surprise the unit sales of PC is going down, when its easy to just replace the motherboard and other hardware and keep the box. Most people don't want many old PC boxes in their homes.