I claim: never.

You are what you are ethnically, that’s it. No amount “living in America” will suddenly make you “American”.

It’s exactly the same for Russians for example, but in Russian we have 2 words to say “Russian”, one of them implies ethnicity and the other one implies citizenship. You obviously can become “xxx citizen”, but you never become the ethnicity, unless you already are.

Because I don’t think we can speak of “US ethnicity”, hence only “citizenship” remains. As such, anyone who becomes a citizen automatically looses “immigrant” status, even if only after a couple of years.