Really, this is a hard to answer question as everyone's situation is fairly different.
Since we are talking about a few different certifications, I'll compare. Some folks are saying that the CISSP is harder than the CISA. Some folks are saying they studied one area and less on another.
It's all based on individual experience, and how you learn. And, how well you can determine where your weak points are and brush up there.
In my case, I studied for the CISSP exam for a week, and then took the test on the weekend, finishing in just under three hours. Now, that makes it sound easy. Mind you, I had been working in the field with a wide variety of positions and experience for about 17 years when I sat for that exam. So, while I won't say the CISSP exam was easy, I just knew the material from my prior education and experience.
When I took the CISA, I thought that exam was a bit harder. But, my time around the audit world was not nearly as extensive as the topics covered for the CISSP. I studied for several hours a week for about three months.
Each person is different, and each person's journey may not be the same.
The Sybex book is big. But, it is good. Also, I second the Eric Conrad suggestion. I also really like his 11th hour book.
Take a look at where you are weak, then take a look at how many pages you need to study, figure out how long it would take you, add a buffer because "life happens" and then go from there.
That is my suggestion. YMMV.
This is an expensive test specially when you convert it to Indian Rupee like in my case. So I had to pass at the first attempt. I studied the Sybex book, for around 2 months (around 20 hrs a week) then solved that Sybex Questions to have pretty good idea in which domains I was lagging, revised the book again, I knew which portions to study more.
I took some of the Seniors' advice from here and just relaxed from the day before exam-day, even tried to meditate. This was probably the best advice I can pass on, as when I was giving the exam, my cool head and my experience mattered much more than the last three months' study.
Then when the Exam coordinator was passing me the single page document, I knew I passed.