Ball pythons have major growth within the first 3 years of life; you will see the most dramatic growth spurts during this time. After 3 years they continue to grow but at a much slower rate. Most people prefer feeding rats and if your snake could make the switch I would personally do so; but after three years of feeding on mice it may be hooked on them. You want to feed 10-15% of the snakes body weight every week. Some people keep their adults on a 10 day as opposed to weekly feeding.