China citizens hold American green cards. How long can their parents stay in America every year?

There are no specific regulations. When parents come to visit relatives, the visa is generally valid for one year, but the I-94 obtained at the time of entry is generally valid for only half a year. Moreover, if you really stay in the United States until it expires and then apply for a visa to come to the United States soon, you may be suspected of having an immigration tendency. Generally speaking, once every two or three months, not more than once or twice a year, will be more smooth.