Ah, very interesting question. I prefer Macintosh because you can keep on using them for a long time. They're like Japanese cars. Most people hang on to their old Macs because the computer is usually still working when they decide to upgrade. I would say for a Mac you should upgrade after about 9 years for sure. I know a lot of people who are still using 9 year old Macs. On a PC it's about 5 or 6 years. Depending on the build quality of your particular PC you can get longer out of it but that depends on how advanced you are with computers. If you are a business and need to turn around the computers for business and you work in a technology field, most businesses I'm familiar with in the Silicon Valley would swap out a portion or all of their computers every 3 years as a rule of thumb - but that's the Silicon Valley from back in the day. Others may have other opinions and experiences. The 3 year rule goes for both PCs and Macs. Some schools buy computers that are one year old and still swap them out every three years to save money.