The ideal length of a book summary depends on your reading frequency and goals. See the following considerations.
Goal: To decide whether to buy a book — For this purpose a few sentences or paragraphs may be all you need to decide whether the topic interests you enough to buy the book.
Goal: To be inspired by information and ideas — A longer summary is best if you want to learn what the author means or key topics without reading the entire book.
Goal: To speedily digest books — Some people want to read voraciously but have limited time blocks. A number of services specify how long their summaries take to consume. Whether reading, listening to book summaries while on a coffee break at your job, or using an app between appointments or at home — focus on a speedy reading experience.