As is often the case when answering an exercise-related question, the answer is “it depends.” The right answer for you depends on your specific, individual needs and goals. I’ll give you some general guidelines to make the decision about your frequency and duration a little easier.
The 2008 Physical Activity Guidelines for Americans suggest that adults should participate in structured cardiorespiratory-related physical activity at a moderate intensity for at least 150 minutes per week or a vigorous intensity for at least 75 minutes per week to experience the health benefits of exercise. For novice exercisers, improving on cardiovascular fitness should be addressed in a twofold manner. The first goal is to gradually increase exercise duration. This allows the body to adapt to the new demands of exercise and respond accordingly to the physiological stress of training. Initially, training volume can be increased by 10 to 20% per week, until the desired training volume is achieved. For example, if you start out exercising 20 minutes at a time for 3 days per week, increase the duration of your sessions by 5 minutes each week until you have reached at least 30 minutes of continuous exercise without stopping. Then, gradually increase your days per week to 5 days, and you will meet the minimum requirement for health benefits (150 minutes per week of moderate-intensity exercise).
For those who already have a solid cardiorespiratory training base, the second phase of training focuses on increasing exercise intensity, in an effort to increase VO2max. As long as there are no contraindications to higher-intensity training, it is appropriate to incorporate moderate-intensity steady-state training as well as interval training. Adding high-intensity intervals to the existing aerobic sessions a couple of days per week increases the time spent in the vigorous-intensity zone, so the frequency of workouts can be decreased from 5 to 3 or 4. The duration of the training sessions can also be reduced. For example, accumulating 25 minutes of vigorous-intensity exercise 3 days per week is all that is required to achieve health benefits. The higher intensity training will also result in greater fitness gains, which apply to all areas of life (e.g., activities of daily living, recreation/sports performance).
Ultimately, the intensity of the exercise session dictates the duration of the workout. The harder you exert yourself, the less time you have to spend doing it. If you decide to try higher intensity training, be sure to give yourself adequate days of rest between workouts (2 to 3 days) so that your body will respond by getting more fit, rather than breaking down from overuse.