There are many different ways to approach cardiovascular exercise. The best approach depends on what your goals are. In other words, if you are a de-conditioned person who is looking to improve your overall health, then your approach would be far different than an athlete looking to enhance sports performance.
Exercise intensity and duration for those who are de-conditioned will start at lower levels. A good way to measure exercise intensity is by a scale known as RPE or rate of perceived exertion. Basically this is an intensity scale rating from 1 to 10. Beginners should aim for workouts that are maintaining an intensity of about a 5 or 6 for 15 to 20 minutes, 2 to 3 times per week. The goal would be to increase duration, intensity, and frequency gradually. This can be self regulating depending upon how this program is tolerated.
Suppose you are already engaging in an exercise program and looking to improve? A good way to accomplish this would be an interval training program. Interval training involves bouts of exercise that are much higher on the RPE scale followed by much easier recovery bouts. For example, after a warm-up you might hit numbers 8 or 9 on the RPE scale for very short periods of time followed by a number 3 or 4 to recover.
Interval training can take you out of your comfort zone and cause you to work at a much higher level. This will help you eventually increase cardiovascular capacity and enable you to sustain more advanced intensities for longer periods of time. Interval training can help your lower intensity longer duration workouts by increasing the capacity of what is considered low intensity.
A program that combines both of these methods is most effective. Varying cardiovascular intensities will not only make you more fit, but will also prevent boredom and overuse injuries. This will lead to a more well rounded program and a more fit individual.