From the JavaRanch SCEA FAQ:
How long do I have to complete the assignment?
You have an unlimited time to complete the assignment. The date on your voucher for the assignment has no meaning. If you buy your voucher for the Essay portion of the SCEA, then you will have one year from the date of the Essay Voucher to take the Essay exam. The Essay portion voucher is not related to the assignment. So please don't ever think that you have a year to complete your assignment, because that is untrue.