Depends on your working experience with EJB3 and JPA, and also how many hours a day you can spend on it. If you can spend 8 hours a day on studying, then yes you can easily do it within 15 days.
I did the exam today, got 88%.
This is how I prepared for it:
First I read the O'Reilly EJB 3 book during a month after work (maximum 2 hours a day).
Then I did the first mock test of Enthuware's test suite and I got 57%. (failed)
The last 3 days before the exam I studied hard from 8 to 5 using Enthuware test suite and Mikalai Zaikin's guide. This boosted my test scores to > 70%. On the final test I got 81% and as I said on the actual exam 88%. Enthuware is a real help! It's worth the money!!!