How much cardio exercise do I need to improve my fitness level?

It is important to consider the goal behind doing cardio. If your goal is achieve and maintain general health, participate in general activities for an accumulated 30 minutes per day at a moderate intensity 5-7 days per week. Moderate intensity here being defined as any activity that increases your heart and breathing rates.
If your goal is to increase your cardiorespiratory fitness, that is, increase the work capacity, efficiency and performance of your heart and lungs; then work at 60-90% (moderate to high) intensity for 20-60 minutes per day 3-5 days per week.
Remember, if you are new to cardio, the point is to simply just move more every day! Don’t get too caught up in the details just yet and don’t do too much too soon. And the more you enjoy the activity, the more you’ll do it so be sure to look for cardio that you enjoy.

Any cardio exercise will improve your fitness level if you are as out of shape as most of us are, but you need to keep doing a little more cardio exercise three times a week at a little more intense level to keep improving your fitness level.

To become more cardiovascularly trained, the American College of Sports Medicine (ACSM) recommends a combination of moderate and vigorous physical activity for approximately 150 minutes a week. (If the majority of the activity is vigorous, 75 minutes a week is sufficient).

The cardio guidelines for improved fitness levels include performing your chosen cardio activity for 20-60 mins at an intensity of 60-90% maximal heart rate (220-age), 3-5 days a week. Build a solid cardio foundation first by training at an intensity of 60-75% maximal heart rate for 30-60 minutes. Then work toward increasing the intensity of your workouts by adding interval training into your cardio program.