일단은 전문가의 글을 한번 읽어 보셔요.
영주권 받을시 Notice를 보시면, 다음과 같이 되어 있습니다. 6개월은 근거가 없어 보이는데요.
If you are going to travel and believe you may be outside of the United States for a year or more, you should apply for a reentry permit. You must be phisically present in the United States when you file the reentry permit application and complete the biometrics services requirement.