How much physical activity should one perform in a day?

That depends on what your goals are. To receive the benefits of decreasing your relative risk for certain chronic diseases, Federal physical activity guidelines advise 150 minutes of moderate intensity aerobic exercise (brisk walking for example) per a week. Or 75 minutes of vigorous aerobic exercise per a week, or an equivalent combination of both. Source: www.healthypeople.gov

However, if you are looking to lose weight, strengthen muscle or achieve some other health objective, you will probably need to go beyond these minimal standards by increasing the duration of exercise and incorporate other components such as resistance training. The take home message is to move more and seek out opportunities to be more physically active.

Studies show that to maximize health and fitness people should engage in at least 60 minutes a day of physical activity. Research has shown that people who engage in 60 minutes a day of moderate to vigorous exercise have significantly lower incidence of heart disease, stroke, diabetes, and osteoporosis or thinning of the bones.

The amount of physical activity that one should perform in a day is based on the pre-determined fitness goals of the individual. The goals of one person may require more time, and duration on a daily basis than one who is more physically advanced. 

For general fitness, 60 minutes of moderate physical activity daily is important. This is to simply keep your blood flowing through your body and reduce your risk of many physical problems. However if you want to reach a specific goal you may need to add on to this.

Physical activity can be as simple as walking or as complex as high intensity interval training. Research as well as experience can help you decide one what you like to do and how often you want to do it. There is a general principle used in the fitness industry called the "F.I.T.T.E."  (FIGHT) principle. You can adjust your physical activity based on Frequency, Intensity, Time, Type and Enjoyment. I like to start from the back of the list. Find something you enjoy. Then figure out what type of equipment you will need for this. Then you can find out the time it takes you to enjoy this activity by experiencing it. You might find out you like to do a certain activity at a moderate level but if it gets too intense you don't enjoy it anymore. So, you will then know your intensity and you can base your frequency off of your intensity. A more intense activity needs less frequency and a less intense activity might require more frequency to reach your personal goal.