The visa is valid for ten years. How long can I return to China after staying in the United States for half a year?

There is no time limit before you can go back. In other words, the visa for visiting relatives in the United States is valid for ten years, and you can return to the United States at any time after returning to China for half a year.

United States immigration regulations:

American visa holders can enter the United States at any time during the validity period of the visa (including the expiration date). US Customs and Border Protection officials will decide how long the visa holder will stay in the United States after arriving in the United States. Even if the visa expires in the United States, there is no problem, as long as it does not exceed the stay time stipulated by the above officials.

Please think about the reasons for re-entering the United States in advance, and don't be caught by the immigration officer.