Parents green card application. Please help. Thanks in adv

Several questions:

1. My parents are here visiting us with B2 visa. They have been here for 3 months(exactly 3 months today). Is it OK to start green card application for them?

2. When they need to have physical examination? Right now or wait for Immigration office's notice? It seems some people send I130,I485 with physical exam form(from doctor).

3. Currently, how long it takes to get GC? Do you think they can get GC before May next year when we are going back China for vacation?

4. What's the main reason for the Immigration office to reject parents' GC applicaiton?

Thanks a lot.