The average person should be practicing strength training 3-4 times a week. It is, however, the intensity of your workout that will determine how often you should be training. As a general rule, train each muscle group twice a week, but if you are working muscles to complete exhaustion, you may need to limit sessions to once a week. Remember to keep a day between muscle groups to prevent over-training. Abdomen exercises can be allotted to both your upper and lower training sessions, therefore practiced daily.
As a general rule, you also need to take off one full day a week from all forms of exercise.