I'm currently going through the application process for pilot but the start will be similar. It took 6 weeks for my application to be reviewed, I was then given a date for my p2 presentation which was about another month. So up to the P2 presentation it could take you 10 weeks from applying. However, the aptitude testing comes around quickly as the CBAT seems to be done twice a month, not too sure about the AST. I've been told by the recruitment team that it could take 3-9 months for the whole process.
When to apply? I can only echo what drewski has said, do the exams first then start your application.