to save about 80% of the time, ONLY revise what's covered in an exam. I know its good to have a general knowledge and understanding of the topic but excessive information (unless it is strictly related to the question) doesn't impress examiners (who are limited in what they can ward)
the topics should be on a syllabus, tackle them first, alot of book pages you don't need to revise. revise smarter not harder!