I started cave instruction with 6 straight days, the first one being all on land. At the end, my instructor flat out admitted that he was really tired. Me, too. I'm glad we did not go beyond that.
Once I was with a travel group of divers, one of whom did a week of cave instruction. Several others in our group were diving out of the same shop, so I saw her from time to time. The class was in session all day each day. At the end of one day, I watched her class. The instructor was droning on about the physical structure of the isolation manifold, explaining why it worked the way it did. (I guess the purpose was to prepare them for the day they would have to build their own. You never know when that is going to happen, so you have to be ready.) I looked at the glazed eyes of the students and thought, "They aren't getting a damned thing out of this."
I really think there is a limit to how hard and how long you can work without the law of diminishing returns coming into play. When you go past a certain point, continued instruction is not only useless, it does more harm than good, because the student will forget so much that they would have known more if they had quit earlier. That is the idea behind the "less is more" philosophy--students learn more in a given time period if you don't overwhelm them with too much.
