We are each different, and with different goals. Take me the writer for example, I want Rugby player legs, that means I need to abuse them 2 to 3 times a week. That doesn't include my upper body, which will get a visit from the muscle fairy twice a week also. But if you are just starting out and trying to add new positive habit in your life, getting active twice a week is great. Your body adapts when you rest, and at this stage we don't want to over-train and pick up an injury. In months 4 onwards, your new found love of fitness may see you exercising more anyway.