I am an Indian and I got a family-based immigrant visa. We reached the USA on 28th May 2015. At that time I was working in Bahrain and I didn't resign from the job and my daughter was studying in India so we both left the USA on June 25th without getting a Re-entry Permit. Now I am in Bahrain and my daughter is in India. How long I can stay in Bahrain or when we can enter USA without any problem.
Assuming you're now a green card holder (not entirely clear from your... question?), you can stay outside of the US for a period of up to 1 year without the re-entry permit.
However, you'll need a good explanation for your immediate and prolong stay outside of the country. And no, "I have a job in Bahrain" won't do it. You're supposed to move your life to the US once you got your green card, or it will be canceled.