Labor Substitution does not take 6/8 months. Basics of Labor Substitution is that a Company say X has an approved Labor Certificate of Employee Y who is no longer working for company X.
Now you Mr Z join company X and the company decides to give you that labor. The advantage of getting that labor is that you don't have to start doing your GC process right from the beginning (i.e Applying for labor etc).
Also the biggest advantage is that if your labor substitution is approved, you get the Priority Date of that Labor and you jump the queue.
How does labor be substituted ? The attorney of Company X applies for your I-140 with sufficient proof of your academic and experience qualifications, as well as more than sufficient proof about the company being a stable, profit making enterprise which has sufficient profits to pay you. All in all, your profile should pretty much match that of the substitute labor.
Once your I-140 gets approved, that labor substitution is complete. However note that additional proof might still be demanded further when your case is being processed at the I-485 stage.
Now, suppose if the labor that you're getting as substitution has a Priority date of August 2001, then when you get that labor (i.e I-140 gets approved), your GC process too gets that Priority date. This Priority Date is the most important factor when your GC process gets into the third and final stage i.e the I-485.
When the date at the USCIS Processing center where your GC is being processed becomes current, i.e the I-485 processing date of that center matches or goes beyond the Priority Date of your Labor, then your case will get processed by that center.
Currently TSC (Texas) where most I-485's are being processed is having a processing date of March 2001. So in the example above, when the date becomes August 2001 or more, your case gets processed. If Labor substitution was not done, then your Priority date will be sometime in 2006 or 2007 and thus you can imagine the period in terms of years that it will take for you to get your GC.
Note that do not get under the assumption that the processing date progresses monthly. It sometimes takes years for the date to progress even by a few months.
However Labor Substitution is not an easy task. There are hundreds of reasons why labor substitution cannot be used effectively. For eg, your academic qualifications and experience should pretty much match with the labor, your company should be profit making and make good money to be able to prove that they can pay you above the min wages throughout the GC process etc etc.....
To answer your other question, you will not be able to apply for I-485 until your I-140 gets approved AND after that when the Processing Date becomes current for your Priority date.
I hope this makes things pretty clear. For additional information, spend sometime going through the threads of this wonderful board.