It depends on whether you are counting TV movies.
Real feature length films that would be released in theaters probably would have been longest if there was a break in the middle of the picture. If you aren't counting the entr-acte (the intermission, which was usually just a musical section with a single picture on the screen, so people could use the facilities, buy more snacks, walk and move about a bit), then you'd have to obviously count that extra time period.
There are a lot of movies made with Americans in them by a mostly American crew, director, production company, etc, but may have been shot partly or mostly/all abroad (outside of the US).
If you want to be certain you could go on imdb dot com and look around for info on where the film might have been made. They usually include the length of time, etc.
I would say Gone With The Wind. However, that's assuming you aren't looking for extras like the above. I'm not sure if Westside Story would count, though I'm pretty sure it was a completely domestic product (and it would include an intermission if I'm remembering correctly). A lot of musicals by Rogers & Hammerstein might count if they were made in the USA entirely, including Sound of Music.
Once Upon a Time in America (a beautiful film)
The Iceman Cometh (starring Lee Marvin)
Gods and Generals (so boring)
As far as some of the longest movies made outside or mostly outside of the USA, I would use these to compare:
Cleopatra (Elizabeth Taylor), Hamlet (Kennethh Brannagh).