The max amount of time you can spend exercising per day largely depends on your recovery periods and your nutrient intake. With too much exercise and not enough fuel, exercising for long amounts of time can be detrimental to your health. You should base the length of your workouts on your goals. As a general rule, your workouts should be between 30 and 90 minutes long per day.
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Dangers of Too Much Exercise
Exercise comes with myriad benefits and advantages, yet too much of a good thing isn't always good. Too much exercise can lead to overtraining, which can cause insomnia, loss of appetite, fatigue, depression and mood changes. Over-exercising can also decrease bone density and estrogen production in women. Those who suffer from over-exercising may also suffer from mental illness and eating disorders.
How Much Is Too Much?
During a workout, your muscles use carbohydrates for fuel, which typically arrives in the form of glycogen. Repeated long duration training can result in chronically low glycogen stores. Longer duration workouts can also result in the release of greater amounts of cortisol, which can detract from one's fitness goals.
Training For An Event
Oftentimes training for a marathon or triathlon - or other competitive athletics (usually under the supervision of coaches) - will require you to work out for more than 90 minutes. This should generally be the only exception to the rule. In order to avoid fatigue during such longer duration training, you will need to refuel during your workout with appropriate carbohydrate and electrolyte sources during as well as before and after exercise. (This is less applicable to recreational exercisers.)
Benefits of Recovery
Training at such long durations and at higher intensities means it's even more important to have recovery days. Rest days are just as important as the days you work out because they allow your muscles to repair and gear up for your next workout. When they don't have a chance to rest, they will fatigue faster, your performance will decrease and you will risk overtraining syndrome. At a minimum, rest one day per week, and if you are strength training, be sure to give your muscles a day or two of rest before you work them again.