1. Labor certification/PERM (proof there is a need by the company) 2. I-140 application (proof you fill that need) 3. I-485 (formal application to receive PR)
It's been a few years since I went through the process, but basically, as soon as you've completed step 1, you can apply for steps 2 and 3. Once you've applied for 2 and 3, you receive an EAD card (work authorization).
It's always a little dicey to switch jobs with just the EAD because employment-based greens card require that you _intended_ to work for the employer who sponsored you. If you leave right after getting the EAD, your application may be denied because they don't believe you ever intended to work for your sponsor.
This is actually a pretty sweet position to be in because the company that hires you only has to keep applying for an EAD card for you. Pretty simple and not a lot of money.
Typically, if you wait 6 months or so after sending in the I-140/I-485, you can switch companies (through AC21). However, you need to find another job that is substantially similar to the one that was the basis of your green card application, you can't just take any job you want.
However, since the labor certification part (PERM) is a much quicker process and you can file for your I-140 and I-485 concurrently, it's not unreasonable to assume that from the date you first start the process, you'll likely to be able to switch jobs after 2 years or so.
This is actually really important for those poor souls from India and China who have priority dates that still in the 90s and won't likely get the physical green card for at least 5 years if not longer.
One thing to keep in mind is that lawyers often disagree as to how to interpret the USCIS regulations. I've had lawyers say "Jesus! Don't even leave the country!" and others say "Yeah, it's fine to switch jobs, no problem." It just depends on how risk adverse they are.
