My notebook is 2.5 years old and I think our desktop is pushing 3, which is very unusual for us, usually it's about ever 24 months for the notebook and 18 months for the desktop, but we really haven't had the need to upgrade as we are not struggling for performance at the moment (software really hasn't caught up to the fastest processors).
That said, my partner got an iPad for Christmas, so I guess that's a new computer in a way (well, really a big iPod Touch).