Sometimes you don’t need so many words to communicate. Communication happens faster, but when written it becomes longer.
In those days wars would go on for many days. Mahabharata war also went on for many days and the preparations were for many days. So it is possible that Lord Krishna spent one whole afternoon in brainwashing Arjuna.
Since Arjuna thought he knew everything, Lord Krishna had to talk to him and convince him in many different ways. So it is not an impossibility.