40 hours for the main story is pretty solid. Extras could take it up to potentially a lot more I imagine.
You can tell the main game will be more than forty because he already stated the demo is 1 hour for the story and 3-4 hours if you do additional content. So if you take this quote into account:
"I’m assuming that the actual game will take about 40 hours to clear, so comparing it in simple terms, the demo is about 1/40th of the actual game’s content. (Laughs)''
That either means FFXV takes 160 hours (doubtful) or 40 for main story alone.