The one subject you simply must leave ample study time for is Flight Planning. Unless you're incredibly gifted, there will be a limit to how much you can achieve in a day before your brain turns to mush. For me, that was about 6 hours, or 2 practice exams.
With other subjects, such as Systems, you can study as much as you like in a day without becoming "overloaded" (this is how I found it, anyway).
I'd personally budget 4-5 weeks for Flight Planning, and a good 4 for Systems. I found Performance and Nav fairly straightforward, and comfortably studied for both in around 3 weeks each. This is all assuming, of course, that you're not working back to back 12 hour days while you're preparing for the exams.