very much depends on what you put in or what need to put in the hardware.
if, say you install a complete current OS with current openoffice, etc...
and you do not need to upgrade these software every few months, or you can wisely choose 'light' apps for the hardware,
then your hardware can last very long.
personally, right now, i'm using over 10 years old pentium 4.
i even still have and using pentium 2 and pentium 3 hardware daily!
choose your software wisely and you can safely use any hardware over 10 years.
coz, good software do not always demand fastest cpu and huge ram to run.
off course, there're exception, like 3d apps, compilation machine, etc that constantly need fastest cpu and huge ram.
but for regular office usages (ms office/openoffice/etc) any good hardware can last as long as the 'real life' of the hardware.
i use it till it died!