Hello Jin,
I have TWO recommendations for you:
1. When to fill in the PMP Exam Application
I recommend that you fill in the exam application as soon as you meet all the requirements. At the minimum, you need 35 hours of PM training and 36 months of experience as a PM. Please read the PMP Exam Content Outline (pages 11 and 12) for the details.
2. When to register for the actual exam
Once you filled in the application and paid PMI, then you can register for the actual exam. My recommendation: Use the PrepCast Exam Simulator and if you answer at least 75% of questions correctly (on average) then you can register. This is no guarantee, but many people say that this is a very good marker. When you register for the exam, you can select the date and time, so you know exactly when your exam will take place.