It's really hard to answer these questions because there are so many variables.
What's your home situation?
If you're living with your parents, and you're not expected to be involved much with the upkeep of the house, you can plan for a shorter studying period. However, if you are a homeowner, with a wife and kids, you can kiss a lot of that free time good-bye, and expect for a longer studying period.
Do you work?
Obviously, if you are working a full-time job, you're going to have to have a longer studying time period. Working 40 hours per week plus commuting time will leave little time to be studying on your own, plus, you might be expected to put in overtime. It also depends on if you're working in an accounting or financial field. If you're working in a non-related field to support yourself, it might not help for the exam. However, working as an accountant, a tax preparer, or even in IT can help you in applicable areas. However, make sure not to consider work experience as a substitute for studying. You need to answer questions they way the AICPA expects them to be answered, not necessarily how they would be in the real world. And plus, most fields require much specialization, while the CPA exam requires basic knowledge of everything that you could imagine in accounting, that you'll probably never use, or care to know.
How well has your educational program prepared you?
It's no surprise that candidates who graduate from great programs, and that take advantage of the great programs can review more quickly and easily than other candidates. I'd say if you gradated from a school with a great reputation and you felt like did a great learning the material (be honest with yourself, look at your grades) you can shorten the study time. However, if it has been a while since you've graduated, you'll want to really focus on the review (or learning) process, and extend the time period. Also, if your program relied heavily on curves, take-home exams, online-quizzes, and “class participation” to pass their students, you'll want to consider really learning the material. There's no way to fake this exam. While it may not be rocket science, it's you against the questions, not “having the right idea and showing up.” You need to decide if you're really “reviewing” or if you're “learning it for the first time.”
What kind of a candidate are you?
There's no shame in solely focusing on the exam, and taking 3-4 windows to take the exam while your Big Four friend passes them all in the first windows with 90's. Different people learn at different speeds, have different abilities, and there's nothing wrong with that. Just be honest with yourself, and take the exams at a pace that works for you.
Honestly, I think that the amount of hours that the CPA courses give for you to review are underestimated. Rather than wasting time and money trying to just get by with a 75, really try to learn the stuff, and get it done as soon as possible. In a “perfect world” where you have no job or other commitments, I'd say 4 weeks minimum for BEC, 5 weeks for AUD and REG, and 6 weeks for FAR. Don't try to cute and prepare for sections simultaneously. Focus on one at a time, take it without worrying about the results, and move onto the next one. With other commitments, that time period appropriately needs to be adjusted.
.