It depends on your primary goals, what you want to do. Moderate exercise burns fat....vigorous exercise only burns carbs. I don't see anything wrong with exercise 7 days a week. I use a bicycle for transportation and don't feel right on days when I don't ride at least 4 miles....this is pretty moderate exercise unless you're peddling hard and going uphill a lot. There's nothing wrong with more intense exercise, but it won't burn fat and it's not necessary to stay in good shape. Your energy levels will vary depending on many factors.
Well, this isn't really true. Moderate exercise burns fat. Vigorous exercise burns MUCH more fat. Clarence Bass, an expert on fitness for older people, wrote several layman articles on this subject. He summarizes the scientific evidence, the studies by Tabata and by Tremblay concerning how different levels of exercise burn fat:
and how different levels of exercise increase fitness:
and an article on the subject, with a quote from Jeff Novick:
Keep in mind, though, that the vigorous exercise approach that Clarence Bass advocates, is not for people who are just beginning an exercise program after years of being sedentary. High intensity workouts should be gradually eased into, not suddenly jumped into.