Applying for permanent residence is the goal of most foreign nationals who first arrive in to the United States as a temporary worker with a visa.  Since employer sponsored permanent residence processing is quite lengthy and lay last upto 10 or 12 years, most foreign nationals are getting wiser and planning ahead to prevent a break in their employment. 

Non-immigrants' stay in the United States is for a limited period.  H1b employees or those in L1 and L2 and many non-immigrant worker status can legally work in the United States only for limited number of years.  However, with employer sponsorship of green card, employee's h1b status can be extended until processing of the green card is complete.

Once, the employee reaches a stable non-immigrant status it is a good idea to start the permanent residence application.  Frequently, foreign nationals ask what is the period they should wait before they can talk to their employer for sponsoring their green card.  There is no minimum period for which the h1b worker should have worked for the employer before starting the green card processing. 

A new employee may be hesitant to ask the employer to intiate green card processing immediately upon onboarding; ideally it should be negotiated as part of the contract for employment.  But more important point is how valuable the employee, the position and job is to the employer. 

It is better to start early because of the lengthy processing times in obtaining the green card.