About ten years ago design guru Donald Norman posted a relatively short article on How To Write an Effective Manual. That post concludes with Norman's Law:
The number of readers is inversely proportional to the square of the length of the document.Norman was dealing with technical writing. He noted, for instance, that people don't actually want to read manuals, they simply want to find the information they need. Since most manuals cover considerably more details than the user will ever really need or want, their length is not only daunting, it's counterproductive. Though Norman was dealing with manuals, it's a good guess that his "law" holds for other sorts of writing as well.