I've just passed the 1z0-895. Here you are my hints on what I've done while studying:
- Read Ivan Krizsan's OCPEJBD 6 study notes.
- Program my own version of all the examples into Ivan Krizsan's notes (avoid to copy and paste as much as possible if you really wanna learn).
- Buy Enthuware preparation SW and do tons of tests. Take notes of the more complicated questions.
- Read Frits Walraven OCPEJBD 6 EJB 3.1 notes. Specially useful when the exam is close and you need to refresh all the concepts.
- Re-read all the notes about the Enthuware's complicated questions.
When having doubts and not finding a clear answer for them, I strongly recommend to program your own code and make your own conclusions (this is the best way of learning and fixing concepts).
I spent quite a long time preparing myself for the exam since I was not 100% dedicated to it, but in my opinion, 3 months should more than enough if you study after work for around 45 min / 1 hour a day.