It’s sometimes an important question for a course/presentation designer: “How long is the average attention span?” or “How long can a person stay focused on a topic?”
It’s a practical question, and you can make important decisions based on that elusive answer. But stay on the question long enough and you’ll see that there isn’t an answer. “How long is the average attention span?” It depends—on too many things.
What makes traditional, pre-1990s books so different from today’s popular books and e-books? To generalise somewhat, the presentation differs in almost every way: Paragraph structure, bulleted lists, visual elements, boxed islands of information, and so forth.