Just wondering how long out of a country you have to be to stop being of that country.
Its not just the passage of time, it's got to be about how you choose to present yourself, and, I'm sorry to have to add, how people respond to you.
My sister was born on the island of Ireland in the '50s, both parents also born there. I was born in London during their first 'foreign' posting. We both grew up in international (expat) communities in cities in Germany.
Sister returned to England to train to be a teacher in '70s with a british passport. Anti-Irish sentiment was strong during the bombings, few people in England have ever understood the different Irish communities. Sister kept her head down, no one who knows her as an adult would have any idea of her Irish heritage. I think her colouring (dark hair, contrasting very pale skin) is obvious.
I went to University in Germany, was delighted to discover an monastery close by, founded by Irish monks, who are credited with the reintroduction of Christianity to Europe following the dark ages. Lots of freckles, sadly not red hair: both British/Irish passports. Still don't enjoy answering the question because it, yes, it really is complicated for anyone whose heritage, birthplace and upbringing are not all the same
People who say there is nothing to make a fuss about, if you are born and brought up in 'here' have, I would guess, not had the experience of being told but 'you are not from here' with the implicit you are not like us and not as good as us.
It is very noticeable that in 'international' communities this is a question which is hardly given much attention, and can just as easily be answered by stating the location of the last posting. Except perhaps when the question is about the impact on any children. It is still the case that you have to for example be born in Yorkshire to play cricket for Yorkshire or born in the USA to be president.
For day to day purposes, while people will notice colouring, in my experience their response to anyone will be most strongly to the sound of their voice, or accent. People who 'choose' to retain an original accent will be accepted of that origin.
Getting back to the original point, it's not just passage of time that determines when you might be considered of the 'new' country, it's how you present yourself in that new country, to stand out of blend in, especially between the UK and Ireland where the differences between people are not especially noticeable unless you are looking for them.
The Celebrities mentioned up thread provide a good example of how the voice works effortlessly to proclaim the heritage.