Hi Hafeez,
The SCJD can be challenging depending on your background and goal. The assignment is really Swing and Socket/RMI. Everything else comes down to common sense and your interpretation of the requirements.
Just to correct Roel's post about the time limit, it has changed to 1 year for the assignment. I know this because I started mine just before Xmas 08 and I got 1-year limit.
I suggest do some reading on Swing and Socket/RMI. Do some code to
test out how Swing events work. Learn about RMI.
When you buy the voucher, it depends on how much time you have. Doing the assignment during free time or as if it is a full time job is totally different depends on your background. Hope this helps.