Excuse me, if a China family immigrates to the United States and Japan, how many generations will it take to fully assimilate into an American or Japanese?

The second generation is basically assimilated, and foreigners will regard him/her as an American with China cultural background. After the third generation, he is an American through and through.

But the premise of the above is to actively integrate into American society. People in Chinatown often only deal with Chinese and can never integrate into the mainstream American society.