standard trade mark parts) well maintained personal computer ?

A computer that is well maintained can keep for over 5 years without starting to bog down to much. But computer speed doubles every 2 or so years, so it's often a case of do you really want to keep that old 3Ghz around, when the new 6Ghz+ comes out?

Basically as long as you want it to last. I still have a 286 from the mid-80s (I think) that I use for word processing. It still works perfectly.