The goal is to maintain a healthy weight. The amount of food needed to do this varies, depending on the animal, the food used, the temperature, etc. They need more at warmer temps. At 13+ cm, yours are big enough to feed only every 2-3 days. You can probably give them as much as they want at that feeding. Certain foods will tend to cause obesity (pure meats, fish filet, pinky mice, etc.), so use these sparingly. If they look thin, feed more. The axies should look pleasantly plump.