I would guess that most CPU's become redundant before they fail.
My old, about to be replaced, 8 year old machine has an Intel Pentium 3 in case that's of interest.
Quote: "The amount of time a processor last is based on how much you use the computer verses how much heat is is subjected to during use, the cooler you keep it the better, cleaner the better as well, regular maintenance on your case can help increase the life of all parts."
The only time the case has been cleaned was the time when the GFX card was upgraded (four years ago?) - the chap who did it took one look, told me to stand back and gave the whole thing a good shot of compressed air.
That was quite enough case cleaning and maintenance for me.
Perhaps I'd better take a bit more care of the new machine though.