The amount of times you should exercise per week would be determined by many factors. Current body composition, current physical abilities, age, current nutrition and lifestyle habits would all determine frequency of exercise. What you want/need to do with your body and intensity of exercise would also be an important factor. The more intense an activity, the more recovery time would be needed for another round of intense activity (specifically if the second round is the same or similar activity to the first). Generally speaking, for maximum results, it would be best to do something every day. 3-5 days per week of higher intensity exercise combined with 2-4 days of low intensity (ie. light cardio, flexibility training) exercise.