I was using small numbers because I wanted to avoid what you're describing, about excessive training resulting in less improvement. Training only one hour a week or only three hours a week, you're not going to experience diminishing returns in most cases. If you were, say, training forty hours a week, 8 hours a day, then you would most likely be experiencing diminishing returns and you would want to cut back, otherwise you will most likely burn yourself out and not progress at all and you could very likely hurt yourself. But most people can handle three hours a week, there might be some exceptions such as people with certain medical conditions or people who are really out of shape but the vast majority of people can train three hours a week and not hurt themselves or have diminishing returns.
Of course, how the class is structured and how your training is structured also plays a role. Doing one solid hour is going to produce better results than doing ten minutes here and there for a total of one hour. Doing five hours straight, unless you haven't worked up to it, is not a good idea and will very likely burn you out.