8老师,请问您“If you leave US any time before you get the plastic green card, you use the stamped (endorsed) visa to come back to US. ”,我第一次入境的时间长度有没有什么要求?比如待够半年? 没拿到plastic green card 时,中间有事情离开美国,最长不能超过多长时间必须回美国?