The answer is: It really depends on what you're working for and It depends on your current health status in terms of disease and weight.
The minimal recommended exercise is 150 minutes of moderate intensity exercise per week. (ACSM Guidelines)
But for the average joe with no exercise tests, heart rate estimations, or Vo2max estimations it's hard to know what "Moderate Intensity" means. It's also impossible to tell 2 different people the same value for moderate intensity. Someone extremely over weight is going to have a different "moderate intensity" than someone whose somewhat in shape.
Think of moderate intensity as being able to speak comfortably while exercising. & Vigorous intensity as being out of breath and unable to say more than a few words.
Light intensity would be you're able to sing a song. (The Talk Test)
I myself workout 6 out of 7 days a week for about 30 minutes per day. My workouts range from moderate to vigorous intensity to continue muscle growth, overload and to avoid plateau.
(The following is adapted from the American College of Sport Medicine)
If you are overweight or obese (as 65% of the adult US population currently is), then your goal is weight loss. But let me reiterate once again; Research does not support weight loss with exercise alone. This needs to be a full lifestyle change, INCLUDING DIET.
If you are overweight you should be:
Working out a minimum of 3 days a week, up to 7 days a week.
You should be working at moderate intensity, (able to hold a conversation comfortably)
Workout out for 45-60 minutes per day.
Expending 1200-2500 kcals per week.
Exercise programs should last 6-8 weeks and should be adapted there after to avoid plateau and continue weight loss.
(As program progresses or individuals feel up to it, vigorous exercise can be added 1-2 d/week)
Workouts should include:
Warm up: 5-10 minutes light cardio
Conditioning: 45-60 minutes (weight bearing/aerobic exercise)
Cool down: 5-10 minutes light cardio
Stretching: 10 minutes stretching muscle group exercised.
(There are special considerations if you are diabetic, have coronary artery disease or are hypertensive, email me if you are any of these and I can give you you're recommendations)
The most important thing to consider no matter how physically fit you are, is listening to your body. Understand your limits, but also understand that your body can only go as far as your mind allows it. Working out 7 days a week can be good, if you're not working the same muscle groups every day, if you're fueling your body with enough nutritious food, if you're getting enough sleep and recovery time, and taking the steps to avoid injury.
As always.. feel free to contact me with any questions!