changing employer during employment based green card application

My employer is currently filing a green card application for me.
This green card application is based on my employment with the company and I am currently on a H1B.

It is currently at the stage where my position has just been advertised.
The advertisement was taken down last week so that resumes can be collected and potential candidates reviewed and interviewed. I believe from this we would aim to file documents showing no US person can fill the job in comparison to myself as supporting materials for my green card application.

My questions are:

1. At which stage during the green card application can I transfer jobs? Is that approximately 12 months from the stage I am at now? I know this may be hard to give an estimate but just a rough date given the stage I am at now would be most appreciated e.g. September 2009

2. I am told that you can transfer jobs before the process is complete and you receive the actual green card e.g. 12 months from now. If I did not transfer to another job and simply handed in my notice at that stage to become unemployed for 3 weeks (I plan to go to Europe for 3 weeks and then return to the US to work) could I still get the green card or do I have to remain employed with my current or a new employer?

3. Given the stage I am at, if the application goes smoothly when should I expect to obtain my actual green card?
Is there a way to file and expedite green card applications based on employment?

I would like to move to New York with my girlfriend next year as she will be attending school there. Ideally I’d like to do this having received green card status by then taking a short 3 week break in between so I can take her to Europe.
Thank you ever so much for your help.