Cardiac rehabilitation is recommended for all patients after open heart surgery, coronary artery stenting/angioplasty, or heart attack. Time and again, it has been shown to improve patient survival, as well as other indices of well-being. As you say, every patient is different, so each individual will have a different definition of what constitute "normal activities." For the marathon runner, competing in strenuous races will have to wait until cardiac rehabilitation has been completed. For the person who was sedentary pre-op, "normal activities" such as walking short distances or climbing a flight of stairs may be achievable within a week or two after surgery.