How much programming background do you have? How much Java experience? How much time can you devote to preparation? How quickly are you moving through the book now? How well are you grasping the material?
So how many days? Anywhere between 21 and 365, depending.
"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer sscce.org
My personal experience is this: I spent anywhere from less than a day to 3 days on each chapter depending on the length and complexity. I'm trying to finish my 1st review pass this week, 2nd pass and the master exam next week, then, schedule the real exam.
I've been a developer for 15+ years, but have only some exposure to Java. Having more C++ experience, I was able to grasp the Java concepts fairly easily. I've been heavily reading K&B and practicing since early September. Before that I read Head First Java.
If you have the luxury of not having a deadline to complete the exam, take the time to understand the material. Don't just cram it in the shortest amount of time possible. That won't make you a better Java developer, because you could get bitten on the job by things you forget or don't understand enough.