Textbooks you're surely going to focus on one chapter at at time and you don't want it to take very long to re-read, which most students will likely be doing, especially when you may have hours of problems to solve for a single chapter. You want to be able to re-read without a lengthy commitment compared to the problem-solving.
Regular non-fiction may not be as universally re-read but it would be more often than with fiction.
With fiction or storytelling a half hour is a good max so there will always be a stopping point within reach, with two chapters not being out of the question lots of times. People may not re-read a fiction chapter as they go along very much, but why not make it easier anyway.
Then again people probably like a chapter that takes whole hour or more but is such a page-turner that it seems like a half-hour or less :)