I started off thinking the same way you are - that there was some magical time to change up their feedings and I would be able to tell when they were going to be hungry by looking at a watch or a calendar. Unfortunately, my snakes can't read watches or calendars so I had to pay attention them instead and learn their behaviors. When the little ones started refusing at the shorter intervals (4 days), I started offering them food at my standard interval (7 days).
Hope this helps!
Generally speaking, this tended to happen around the 500 gram mark. Take that with a grain of salt though because I've had them keep pounding away long past that size.