If you want to be fit, you need just 20 minutes a day. The number of calories you burn in these 20 minutes varies with the exercise.
Twenty minutes of walking at a moderate pace will burn 80 to 100 calories, while a high-intensity interval or cardio circuit may burn at most 250 to 300 calories.
If you can spare 20 minutes everyday for moderate exercise, it will stimulate your immune system and make you less prone to diseases like arthritis and fibromyalgia.
Check this 20 minutes workout video that will burn a ridiculous amount of fat.