It would be hard to come up with an answer to your question,and you'd have to define what you meant a little more clearly.
Most people in the USA with English, Scots, Huguenot, Dutch or German roots can usually get back to at least one line that was here before 1776. People tended to marry within their language and religion in the first generation, then within just their religion the second and suceeding generations. (At least Protestants married Protestants, Catholics Catholics and Jews Jews; the distinctions between Lutherans, Presbyterians, Baptists and Methodists dimmed on the frontier.)
As a result, most of us white americans have mixed heritages. I, for example, have a line that goes back to 1623, and another that goes back to 1855. The rest immigrated between those two dates. I have a Huguenot/English couple who married in 1806 and an English/Dutch marriage in 1691.
Hispanics are the fastest-growing minority in the USA today. Some where "here" before 1776, only they were in Mexico; then came the war in 1846 and they were US residents. Some came over last week, legally or not. Whether Hispanics are "White" or not depends on who is asking and what qualifications they use.
The Spanish colonists tended to take Indian wives. If there were BMD records, most Hispanics could trace their ancestry back before Columbus, through that first maternal link.
There have been 7 generations in the 232 years since 1776, using 33 years (the average in my family) for a generation. That is a lot of time to mix family lines.
Well among those of European ancestry it might be higher than people think. As time goes by due to intermarriage, a bit of colonial ancestry is mixed with the immigrant ancestry that came after the country was founded and therefore the colonial ancestry has been spread out.
I am mixed. I have colonials, among them ancestors at the original Jamestown settlement, and immigrants.
I know in the South we have not had a tendency to have white ethnic neighborhoods. I know a guy born and raised in New Jersey that said about 30 years ago, in our area in Southeast Texas, we probably have a better melting pot than in the North. He said up North, various white ethnic groups stay to themselves. We have always had a fairly large Italian population but they have not had a separate neighborhood, except for one street that was for years outside the city limits and was an Italian truck farming area. Back in the days of the family owned neighborhood groceries store, a large portion of them were Italian owned.
When I lived in San Antonio, I lived in what had been and was still to some degree an old Jewish neighborhood, but the reason for that is there was an Orthodox synagogue close by and Orthodox Jews cannot use any means of transportion on the Sabbath. So in order to attend Saboath services at the Synagogue, they had to walk.
We had Conservative and Reform in Beaumont, and they lived among the non Jews. .
Now, they did exclusively marry among themselves for many years. They established a parish for the Italian speaking people when they first came here. Even after many Italians were in other parishes, the old Italian church remains, now the Vietnamese have a Mass there.
It has been told that for years people from one part of Italy would sit on one side of the Church and people from the other part would sit on the other side of the church. The parish was really stunned when a fella and a girl from opposite sides of the church decided to get married. They had a meeting with the parents. Finally, the question that they were really concerned about came up. What side of the church are you going to sit on? They had a marvelous way of handling it. They stated they planned to take turns sitting on one side of the church one Sunday and the other side on the next Sunday.
Now, when you see wedding anouncements in the paper, you find that brides or grooms with Italian names, frequently have grandparents with non Italian names, or brides or grooms with non Italian names have grandparents from Italian families. So among those of Italian ancestry will definitely be those that have colonial ancestry.
The Spanish are considered caucasian or white and many intermarried with the native population. The Spanish colonized old San Antonio back in the early 1700s with families from the Canary Islands. Unless it has changed in the last 40 years, no way were daughters of those colonial Spanish land grant families allowed to intermarry with Mestizos.