How much time you need really depends on how well you know Java, how fast you study and other personal factors, so it is hard to tell exactly how much time you will need. The exam is not really easy however, and most people need at least a few weeks, upto a few months, to study for it. Have a look at the books available for the SCJP - the K&B book (SCJP Study Guide by Kathy Sierra and Bert Bates) for example is 850 pages thick. Can you estimate for yourself how long it would take you to meticulously read that book and learn what's in it in detail?
I think you should not think about the time it takes to prepare for the test.
If you are smart enough you can take less time than others think. So start preparing today and one more thing don't rush for the exam.Take your time as to get a good score as only the certificate must not be your goal.