My parents have had two machines only since forever. One of them is 3 years old, the other a bit older.
One set of in-laws has had a single computer for about 3 years. That was their first. The other picked up one 2 years ago, also their first.
My sisters each have one. Most people that I know who aren't computer hacks only have one. I would suspect that people buy new computers on pretty much the same timescale as a new car.