The first element of cardiovascular exercise is
frequency of the exercise, which refers to the
number of exercise sessions per week. To improve
both cardiovascular fitness and to decrease body fat
or maintain body fat at optimum levels, you should
exercise (cardiovascularly) at least three days a
week. The American College of Sports Medicine
recommends three to five days a week for most out
of shape and/or who are overweight and doing
aerobics class or jogging, might want to have at
least 36 to 48 hours of rest between workouts to
prevent an injury and to promote adequate bone
and joint stress recovery.

Duration of Exercise
The second element of cardiovascular exercise is the
length of time spent exercising. The cardiovascular
session, not including the warm-up and cool-down,
should vary from 20-60 minutes to gain significant
cardiorespiratory and fat burning-benefits. Each time
you do your cardiovascular exercise, try to do at
least 20 minutes or more. Of course, the longer you
go, the more calories and fat you'll "burn" and the
better you'll condition your cardiovascular system.
All beginners, especially those who are out of shape,
should take a very conservative approach and train
at relatively low intensities for 10-25 minutes. As
you get in better shape, you can gradually increase
the duration of time you exercise.