The short answer is that you should change it whenever you want. There are no hard fast rules, and the program will never automatically adjust your Run-Walk-Run ratio for you. As your fitness improves, you can move up to a more intense ratio, but don't get too rigidly locked into a set ratio, you should adjust as needed. Take factors like speed, hills, heat, humidity, time of day, etc. into account and feel free to adjust the lengths of your walk breaks at any time.