The average movie runs between one and half and two hours. The average song length is three to four minutes. Obviously, research has gone into figuring out how long the human attention span is - I'd say it's somewhere between a gold fish and a cat - to determine when an audience tunes out.
Most debut novels are a certain length. YA novels are between 35,000 and 70,000 words. Adult books somewhere between 80,000 and 120,000 words. Obviously publishers have done some research into how long a book should ideally be. But is this a guide that even successful authors should stick to?