These AskHistorians threads may be of interest to you:

• At what point in time did Americans develop an American accent (or accents, for say the north and south)?

• When did the current dialect of American English diverge from traditional British English?

• Would Americans at the time of the revolution have spoken with a distinct non-British accent?

• Before the American revolution, is it true that the British spoke with American accents?

Or maybe you are asking about when more recent European immigrants to America lost their accents?