you should engage in at least 30 minutes of moderate activity each day to realize the health benefits of exercise.You would then repeat the interval cycles for 20 minutes, ultimately achieving 10 minutes of high-intensity exercise and 10 minutes of active rest. This would provide the equivalent benefit of one hour of moderate physical activity.
I have heard that it is important not to exhaust your body out. So 45 minutes to an hr. I know I have done P90x in the past and I believe that the longest exercise that they had was 1 hr and 15 minutes. The yoga may have been 1 hr and 30 minutes. I think it depends on you and your body.
The amount of exercise one can do in a day depends on many different factors.
What types of exercises,? (strength training, cardio, crossfit etc..)
What is your experience level? (training for years, new to training, etc...)
How much rest between exercises? (level of intensity?)
What are the goals? Just to build muscle or increase strength, power, endurance?
Lots of factors go into how long you should exercise to build muscle fast.
Of course Diet and nutrition are key components.
As is rest and sleep.
You could build muscle exercising only 20 minutes a day, but far more than just exercise factor into building muscle.
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I agree with most of the comments. I personally do exercise at least 30-45 minutes per day. I do cardio hip hop which can be a bit intense depending on the music that I'll be dancing to, but of course you need to do some preps first before jumping to the main work out. I believe working out for 30-45 minutes per day is healthier than working out for hours since your muscle tissues can tear up because of over fatigue.
Ever since I've been training and exercising I've only ever gone to the gym 3 or 4 times a week and the other days are rest days so my body can recover. I normally spend between 1 and 2 hours at the gym and make sure that I never go more than 2 days in a row. I know people that go every day and while there's nothing wrong with that I guess depending what routines your doing, I do think that if you go every day then it can actually do more harm than good as your body will become overtired.
I was also told that it's on the rest days, as your muscles repair themselves, that's when you start to become stronger because the rest helps the muscles grow and it's not when your actually training. I'm not 100% sure if that's correct though so maybe somebody on here can comment on that if that's actually right or not?