Certainly not that often. My PC is just about 2 years old, but I had my previous one for about 4 years, and only replaced it because it broke. It wouldn't be worth buying a new PC every two years.
I came across an article online saying that the rate of doubling has slowed to about every 3 years because we are slowly reaching the point where it will be difficult to impossible to miniaturize computer circuitry.
It is interesting what triggers someone to upgrade. Three PCs ago, that one broke so I bought a new one. My last PC I saw cheap components on sale for black friday 3 years ago and built one. Now I'm tempted again!