Hi Naren,
from my own experience;
SCJP
I had 2.5 years industry experience of developing in Java and at first glance there didn't appear to be much for me to learn.
I started studying half a day a week in February for SCJP and quickly found this wasn't enough. I then upped it to 1hr a day, 4 days a week + a half day and I passed the
test in July. Maybe you could do it quicker if you're studying for it full-time.
SCJD
I started the SCJD assignment at the end of February this year (1 month ago) and, surprise, surprise, it is very difficult and time consuming. Since then I have been working on it full time (so 4 weeks @ 39hours a week) and I think I could be between 2 to 4 weeks more. Once I've submitted it I'll sign up for the 2nd part and then I'll need to wait 4-6 weeks for it to be graded.
So in total I'll have spent about 3 months or 12 weeks (based on a 39 hour week) then wait 4-6 weeks. But I must stress that's just coming from my own experience, you need to decide for yourself once you get into it.
Final advice;
don't underestimate the difficulty of either certification,
allow yourself plenty of time to study / do the assigment,
let yourself get into the study / assignment before making any estimates
Regards,
John