After 4 hours a day, your cognition starts to decline, which means for every additional hour you revise, you start seeing diminishing returns as your focus wanes. 4 hours also takes a bit of training to get to. Most people can't do an hour of deep work.
6 hours is probably the maximum you can go but that is if you have trained yourself to focus that long, which can take a few years. You'll probably learn more in the first hour of revision than in the last 5 hours if you do a 9 hour revision day.