回複：有綠卡後，在美住滿2.5年，持綠卡回國定居，每半年來美住一晚，即可永久保持綠卡身份？

If you are lucky. Because you really cannot survive scrutiny that you abandon US residence if you don't have reentry permit. approved N470, or core family members living in US with you filing US tax return when working abroad, etc. If you don't have any of those, you really cannot argue and win when IO wants to cancel your GC. If you are lucky, IO may not even question at all.