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父母5-6年前來過幾次美國，都是循規蹈矩按時回去，也沒有延期過。

父母去年來美國6個月，十月中回國，打算讓他們三月底再來，就是回國呆了5個半月再來。
5個半月入境有風險嗎？

-Should be OK, but need to prepare for some questions such as why you visit US again in such a short period of time.

哪裏可以找到具體規定？-No. Everything depends on the IO at the port of entry.
多謝！