About a qarter of an hour from the start of the movie. When mrs. Ryan is standing in the kitchen and looks up from the counter and sees the army car coming towards the farm. In the background behind the car you can clearly see tracks in the field with aprox. 24 meters between them. At that time they didn't have machines that wide.
And they still don't! 24 metres is about eighty feet! If the tracks in that field were made by a single vehicle, then it was a battleship with a lousy sense of direction and a suspiciously convenient set of wheels. Perhaps it would be more logical to assume it was two harvesters cutting parallel paths through the crops - as they did in those days, and as they still do today?