If I were to be able to go back in time, how far could I go back and still recognize an American accent as it is today?

Plus, two follow-up questions:

What factors caused this shift?

How, when, and why did regional accent develop?

Thank you for taking the me to reply; I really enjoy reading an learning about aspects of history by people that are knowledgable and enthusiastic about history. I've learned so many things hat I never would have ought to ask!