There is no six month magic rule that people often talk about. But the way it works is green card is given based upon the premise that the employee is taking the job on a permanent basis. Permanent does not mean forever. Permanent simply means indefinite basis. And what is important is at least in my view that the intention or the intent at the time of getting the green card approval must be indefinite.

Requirement to Work for GC-Sponsoring Employer
The U.S. government grants employment-based permanent residence (commonly referred to as the “green card”) because an employer has established that there is need for a particular worker. Employment-based green cards generally are based on a future job offer concept, regardless of whether the foreign national is working for the employer / sponsor throughout the processing of the green card case. The employer offers a “permanent” position that is set to begin upon approval of the green card. Similarly, the beneficiary promises to accept that position upon approval of the green card. Thus, such individuals are expected to follow through, working in their sponsored positions for a reasonable duration once they receive green card approval. Failure to do so may create serious problems for these individuals later, particularly if they attempt to naturalize to U.S. citizenship.

The point is that until you receive the green card, you are a temporary worker and the green card is issued on the basis that you will work for the sponsoring company 'permanently' after the green card is granted.