I'm a Luddite. Upgrade when you can't do the stuff you want to do easily. (My 12 yo son just got a new/used computer to replace the 10 yo thinkpad that had a failed battery and a failed touchpad & eraser. I figured it was time. Now he's using a 6 yo thinkpad...)

As tanstaafl said, there's no "right" time - just your time. And time is money...

-jk