I would say that the answer is very open ended and doesn't even have a rough estimate. Depends on so many variables. Storage of bike, quality of cable housing, how the person shifts/brakes, how much the housings are flexed, is the bike cleaned after bad weather?

I can see a 10 inch piece of brand X housing used on a downhill MTB going though mud and everything else failing after a few months whereas that same 10 inch piece used on a comfort bike kept indoors might not fail for 10-20 years.