How long do you keep up with an exercise routine before reaching a conclusion that it just doesn't work?

In order to stay in shape, you need to do cardio (which you are doing right now), strength, and flexibility. You seem to be eating healthy, but some fruits and vegetables wouldn't hurt. If you stretch 5-7 times a week for 10-20 minutes each time and do strength 3-5 times a week, doing exercises in reps and sets (reps=repitition, sets=how many of the exercise in a rep), you should be good. Please answer my question, "Diet/exercise plan for a 14 year old girl?".