The vast majority of North Americans don't even come close to getting the MINIMUM recommended physical activity to maximize health. We are not even talking training level exercise, we are just talking about movement that includes walking!
As you know, this makes me nuts. The number one way to lower your risk of pretty much most physical and mental illnesses is to exercise regularly! But research has shown that even educating people around the risks of being sedentary, and the benefits of exercise does not do much to motivate people nor increase adherence to exercise programs. As cynical as it sounds, I've pretty much accepted that humanity's future is grim when it comes to our health. The only way to really promote lifestyle change of populations is through changing the way we live from the top down (i.e. government, institutions, and business completely changing policies and infrastructure). I don't see this happening. Too many people are profiting off of us getting fatter and sicker.
The problem is, we humans adapt to our environment and look for ways to conserve energy, and we currently live in an environment that encourages consumption of high calorie/low nutrition food and a sedentary lifestyle. If our environment doesn't change, we never will (though some are able to resist the temptations through human agency, but this is not the norm).
Because of all this, over training is not really a major public health issue. It is relevant to a very, very, VERY small segment of the population. It primarily effects high level athletes and those with eating disorder/body image issues who may over exercise to manage their weight.
Anyone educated in fitness/physiology knows the importance of rest in a training program. Not only does it help prevent injury, it is essential to get maximum gains, since muscles rebuild during rest phases, and this is necessary for building strength.
So how much exercise is too much?
The answer is, it depends on numerous factors.
In the fitness industry, we design exercise programs around the FITT principle (in parentheses are examples of each):
F = Frequency (5x week)
I = Intensity (Moderate)
T = Time (30 minutes)
T = Type (cycling)
These variables are key in determining how often is appropriate. Also, it depends on how you define a rest day. Some people only think in extremes, so its either go to the gym and spin and lift weights, or do nothing at all except drink Manhattans and eat tacos. It doesn't have to be so all or nothing!
I believe most people should be trying to exercise every day if, as most do, they work sedentary jobs. But in most cases, it is best to vary your activities. A lot of people who do decide to begin a fitness program tend to focus exclusively on one thing (i.e. cardio, or yoga, etc.). Ideally, you want to incorporate all the components of physical fitness which includes:
One of the reasons people often get injured is not the number of hours they are exercising, but because they are doing too much of one type of exercise, and not cross training. Like even someone training for a marathon can benefit from doing strength training and yoga, instead of doing nothing but running.
Does exercising every day mean going to the gym every day?
In fact, I think its better if you can seamlessly work activity into your day instead. Perhaps you only go to the gym and strength train twice a week. Then one day a week you walk 5km home from work. Perhaps another day you do a yoga video at home. Another day you and a friend go jogging together and the other 2 days you do a 30 minute ab video when you get up in the morning.
Or you never set foot in a gym at all.
So exercising every day can definitely lead to overtraining, if, say, you are doing a spinning class 7 days a week, or running 12km every day. But exercising every day can be extremely beneficial if you have a balanced workout program that varies the FITT variables and has a good balance of physical fitness components.
When it comes to strength training, you should give a muscle 48 hours rest before working it again. But you can still strength train every day, you just work different body parts on different days ensuring sufficient recovery time for each. For example, you do shoulders, back and chest one day, arms and legs the next.