I have sometimes wondered how long a person can be detached from their home country before the local culture has moved on so far

I believe you never do.

My sister left home for the US when she was 18, today is her 54th birthday and although she is american in many ways, she still keeps cultural traits of her years here. Maybe because she comes almost every summer and she has never wanted to let some of the local traditions and habbits go away in all these years. But even though she has many english words in her local catalan or spanish talking, part of her will always be where she was born and raised.