How long can the entry time of American immigrants be delayed at most? What is the procedure?

If you get a visa, you must enter the country within six months.

If the schedule is up but there is no visa, contact the American consulate to explain that you don't want to go to the United States for the time being, then keep in touch once a year and ask the consulate to arrange a visa when you want to go.

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Look at the validity of your visa. The immigrant visa is valid for half a year, and nothing comes later.

You can go through the green card formalities and fly back immediately. The green card will be sent to China by relatives.