Is your intent just to pass the exam and add 5 letters to your resume or to really understand the J2ME environment?
If the former, you can probably cram for a month or so (2-3 hours a day) and clear the exam with decent score, although that's not in the spirit of the exam.
If the latter, it would probably be 2-3 months (2-3 hours a day) with some time for writing some applications in various aspects and understanding how they work.
SCMAD (or any ther SC exam for that matter) is intended to be a
test of how well you understand the topic and not how quickly you can cram for the exam.
Hope you get time to do the latter!