Its a rough figure I've been told - it probably depends on where you are living and the standard of construction. Every component will of course wear at a different speed and some (kitchens and bathrooms especially) sooner. Often roofs need a lot of work after 40 years or so and sometimes cheap walls too. Theres books dedicated to this kind of stuff. See the following link for some examples: http://www.thisoldhouse.com/toh/m/article/0,,216991-2,00.htm...
I hear twenty years as the standard figure for an asphalt-shingle roof. But I live on a street with houses that are in some cases 80 years old and more. And the Washington suburbs are full of bungalows built in haste 60 years ago and still functioning--with a new roof, maybe, and with additions, but with an original core.
Bathrooms and kitchens don't so much wear out as go out of style. The grout gets crummy in the bathroom, the porcelain wears, and then a generation that watches cooking shows doesn't want the galley kitchen and electric stove of 1950.
