Basically, once we Americans began to come to what is now the US, how long did it take is to evolve to our current dialect? I've noticed a lot of movies that take place during the Revolutionary War and even Civil War are split between us still having a somewhat English accent, or sounding basically like the Americans we do now
Not to discourage further responses, but u/keyilan, u/YuunofYork, and u/lord_mayor_of_reddit answered several similar questions