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感谢8老师！我目前绿卡未到期，再请教i751收据大概要多久呢？

-One to two months, case by case.

申请i131不知又要多久呢？

-Case to case. You may file I-131 now even if you don't have the I-751 receipt.  But nromally you need to conduct the biometrics for the I-751 and I-131.  After you doncudct these two biometrics, you can leave US. 

You may request expedited processing for biometrics for the reemtry permit. See: 

How to Get Your Reentry Permit Expedited: 6 Reasons USCIS will Expedite Your Application (ashoorilaw.com)