That is highly dependant on the manner in which the end user uses it.
I have an absolute dinosaur of a desktop.
PII 300mHz processor, 256mb of PC100 RAM, nVidia GE Force II vid card, blah, blah, blah....
It was given to me by my mother and when I acquired it the old gal still had Win95 installed for an OS and the first day I got it I wiped the HD and installed Win98se.
It has been running faithfully without a single reformat (no crashes, no nothing, no anything that might indicate the necessity for it by the way) since. It never gets used for anything more involved than 'Net surfing, emailing, and running WinAmp for music but it chugs along faithfully with NO problems nonetheless.
That was 6 years ago.
By your estimation I reckon I'm seriously overdue for a crash and reformat, eh?