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。请问如果现在为父母申请绿卡同时申请回美证，回美证多久能下来？
-About 3 months, but they may apply for the emergency AP in the local USCIS Office after getting the I-485 receipt if they have the emergency reason/evidence.

如果回美证批下之前是不是不能离开美国？
-Right.

如果拿到回美证之后回国，最多能在中国待多久而不影响绿卡申请？
-AP is valid for one year, and they have to return to US before the AP expires.

Another issue you need to consider is that they need to do the fingerprinting about 2 months after filing I-485, and also, they may required to do the interview and if they are not in US to do it, it will result in the I-485 denial.