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请问8老师， 父母办绿卡的那个无犯罪证明， 因为只证明无犯罪纪录到办证的那一天， 那需要来美国之前多久办？
-The later the better. But they don't need to provide this document for I-485 unless their fingerprinting fails.

多少时间有效？
-No time limit because it is  just the proof  for the period of time their stay in China. If they return to China and stay there for more than 6 months before their file I-485, they need to get a  new one again.

谢谢！