It all depends what you do with your machine people who are into graphics (image/video editing) and or gamers would find themselves upgrading often because as software gets better they become more demanding as if graphics software wasn't demanding enough, personally I upgrade to keep up with tech growth or if I see a good deal and I have the money I'll get it. I know people who do the basics check mail, use office and now and then listen to music people like that really don't need to upgrade for lengthy periods.
Intel i5 2300, Gigabyte Z68XP-UD3, 12GB DDR3 @1600 MHZ, XFX 6870 2gb (Stock). All Stock
For in much wisdom is much grief, and increase of knowledge is increase of sorrow | If you love something let it go, if it comes back to you its yours if it doesn't, it never was.