Howdy, champion!
Well... I think that, if you already have experience with
Java development and feel comfortable with developing a solution for a real problem, then I think you can start with the SCJD development already. Otherwise, you may consider reading the book first. Anyway, even if you already have experience, it would be nice to take a quick look at the book, since its content is great and it certainly is the best reference for this certification.
About the time, it also depends on your experience and the free time you can spend with it. Some people take a month, others take, like, 6 months (like me), others take years...
Cheers, Roberto Perillo
SCJP, SCWCD, SCJD, SCBCD