Theres never a bad time to do the right thing. I would suggest that get the certification as soon as possible. But for that you need to build a strong foundation. You cant build a multi storey building if you dont have a strong foundation. As you are in first year i suppose you might not have much programming experience. If you have then well and good.....If not make sure you do little bit of C++ and load object oriented concepts in your head.
Next start with a beginners book on
java, i would suggest head first java. Study it throughly and get the concepts in your head perfectly.
Next take a SCJP exam guide. First start with Kathy seirra book and do it throughly start to end, dont miss out a single
word. It is one of the best books
After that read Khalid Mughal , it is SCJP bible.
Coding is the key. The more you code the more you are clear with your concepts and it is the key if you want to score 90%+ in the exam
In the end practice with mock exams a lot of them are available in this forum and other sites on net
So i would say 6-7 months of consistent study will be enough. It all depends on how you apply your brain and the concepts.
Dont make haste to give the exam, that is what most new comers do. Take your own time. Dont give the exam until you are perfect with the objectives otherwise you may flunk.
And of course it will help for the placements. If certifications were not important for jobs then they would not exist and no one wud have done it.
And remember you are not learning java just for the SCJP you are learning it because you are interested in it.(thats what the attitude should be)
all the best for SCJP and your engineering career.