The time greatly varies for each person. Manhattan's Number Properties guide contains a lot of important information, so it makes sense that you would spend a significant amount of time on it. Depending on your current level and your target score, you may not need to spend as much time on some of the other guides.

One of the MGMAT instructors has recommended that you read the chapter and take notes, then complete a few of the end of chapter review questions. They go in order of difficulty, so try numbers 3 and 8. If you don't get those correct, you may want to spend more time with the chapter and work on a few more questions. After reading the chapter, you should work on a few of the recommended OG problems. Don't complete all of them, because you'll want to save some for the end of your prep.

Are you planning to apply to b-school this year? Unless you are really pressed for time, then I recommend that you spend as much time on each guide as you need to build a solid mastery of the material. Don't worry about how long other people may spend. It won't help you to rush through the guide without fully understanding the material.