This scene with Jack Nicholson from the 1978 movie "Goin South" seems classic to me. You would have to watch the whole movie to understand the context; but I think there's something touching, brave, human and humorous about this character working in a dusty mine saying "I can do this all day long". It's not clear if he's trying to convince himself or his partner that he can persevere - which seems like a common human condition.
