Hi Raja,
There is no time limit on the assignment. So you can take as much time as you like (one person recently reported that they took over 3 years to complete it).
You must take the essay exam shortly after uploading your completed assignment. Your assignment will not be marked until after you have completed the essay exam. Sun require you to upload the assignment before you sit the written exam.
There is often a time limit on the voucher for the essay exam. So if you purchase the voucher for the exam now, you may find that it expires before you have finished the assignment.
So Nicholas' suggestion is a good one: where possible, purchase the essay exam voucher when you are ready to upload the assignment.
(Note: in some countries this is not possible - some Sun offices require you to purchase the exam voucher at the same time as you purchase the assignment
)
Regards, Andrew