It's really hard to say how much time it will take you to prepare for the SCJP exam, because it depends on how much you already know about Java, how quickly you learn things, how many hours each day you have time for studying and many other factors.
See the JavaRanch SCJP FAQ for useful information about preparing for the SCJP exam.
Well, the advantage of the Sierra and Bates book is that it covers exactly the topics which will be asked in the exam. Nothing more, but definitely nothing less. If you don't have an idea what to do, buy the book, read it carefully and you'll be sufficiently prepared.
Being an experienced Java programmer is often not as helpful as would be expected. Many many commercial and experienced Java programmers do not know the details of some of the topics. For example many people are not aware of the non deterministic nature of Threading.