The longest lasting component I think is either the modem or case...
I have went through so many upgrades where the modem and case have stayed exactly the same... in fact I have never upgraded my case. I bought a modem twice, for two computers, and never upgraded (well, bought a 33.6, upgraded to 56k and 33.6 went in another computer).
Monitors also last long... the 14" I have downstairs is the same 14" I had with my 486, still works. Mice and keyboards don't last as long.. I've replaced several mice and a two keyboards.
The only things that ARE upgraded frequently are motherboard/cpu/ram and not as often but still relatively frequent, the video card. Network card is something I buy once and dont upgrade for a while.