During my time when i brought the voucher in 2009 - there was no time limit in terms of when i need to appear in part 1, part 2 and part 3...i first brought only part 1.
I appeared in part 1 during 2009. But finished part 2 and part 3 in early 2011 only. When i was in a doubt-i sent an email to Sun and got a response that there is no validity related issue.
But i keep on hearing from some folks that there is a validity period(expiry time for the voucher) from Oracle/Sun - may be some folks might have got some discounted vouchers which might be having some strict terms and conditions....but when i got the part 1 voucher in 2009-it has retake facility but no restrictions in terms of validity...later when Sun was acquired by Oracle...i called in to check on this and they were telling me that i need to finish within a year otherwise buy another voucher...i sent the email i got from Sun and then they(Oracle) honored it.....
So when in doubt-check with them.
Ideally one should appear for part 3 within a week. Part 3 is to make sure that it's you who have done part 2 and how you can defend your architecture decisions. If anybody takes more time - it might simply raise a doubt in the evaluator's mind that "hey part 2 is done by somebody else".
i uploaded my assignment and appeared for part 3 on the same day. Some of my friends(who cleared SCEA5) in fact appeared for part 3 first and within a week uploaded the assignment. Remember part 2 and part 3 are always graded together. So it's a matter of convenience sometimes....
Moreover more time gap you have between part 2 and part 3 - the more time it will take to get the end results. And believe me some of my friends who waited for around 3-6 months for their end results were having sleepless nights to hear about their results.