I have a problem with the poll since it asks about one's 'newest or primary PC". Which is it? My primary PC had just passed the 3 year mark. My newest PC which has a very specialized use is less than a year old.
In any event, I figured out a long time ago that buying a mid level PC is the best deal. Buy, very low and I am replacing the PC in only about two years. Buy, to high and my super-duper PC is outclassed in a year by a mid-level PC at 1/2 the price. I replace my PC when it not longer functions in a useful way for me. That seems to be at about four years. LIke my cars, I tend to use them until they are an embarrassment!!