I think it's not so much the number of hours that makes for "too much," but too many hours of frustrating pointless practice where you don't accomplish anything. I know a couple that practices 10 hours a week and they're stiff and miserable and always picking at each other. Perhaps if they scaled back a bit so that when they went to practice they were much fresher mentally then they'd accomplish more. To me, the ideal is for the majority of time to arrive at practice looking forward to dancing with your partner rather than dreading the whole activity. Of course there are ups and downs, I just want the ups to predominate.