It depends on your skills , experience. As I purchased the second part at the beginning of April I even didn't know about the deadline of August (now postponed to October). But as I read it, I firstly thought 4 months are not enough even with my experience of nearly 10 years, as I really planned to spend only some hours each Saturday. So I looked for many advises. But then I told my self : simply start. So I spent 2-3 hours daily after work in the first 2 weeks (I also have family and son) and after 2.5 weeks I have done nearly everything. Then I spent another 1.5 weeks correcting something and writing down some thoughts. After 4 weeks I didn't have something to write and took part 3 on May 6. I think I spent between 45 and 55 hours for part 2 and only few hours for part 3, simply reading this forum preparing myself for the questions
Of course I tried to find a compromize between good assignment and chance to resubmit it till August. You don't have this chance, because it now takes more than 2 month to grade. But if you'll work daily on assignment you'll do it. It's worth doing it, as you won't need any expensive preparation course for upgrades and I hope, that J2EE 6 Certification will be offered soon. I don't think there will be many programmers wanting to be certified paying so much money for the preparation course, so this certification will be even more valuable for sure
It depends on your experience. If you have enough experience, it is plenty of time. Take a look at my blog post where I itemize how much time I spent. If you don't count the weeks I did nothing, I finished in four weeks.