Greencard Processing time

[choice2012]

Hi

I am currently under H1B and recently transferred my H1B to a consultant. My 6th year H1B ends on Feb 24th, 2013. In case my new employer agrees to file my Green Card, how many months prior to my H1B expiration then they start Green card processing.
Also, for how long does it take to put an advertisement and get resumes for my position.

Your valued guidance in this matter is greatly appreciated.

Thanks