Well there are a few things:
1. When the snake gets big enough that I can feed 1 larger rodent every 8 days. ( I feed every 4 days for young snakes and every 8 for older snakes for simplicities sake)
2. When the snake starts refusing meals on a 4 day schedule.
3. How the snake reacts to larger rodents. Basically I go from feeding multiple rat pups to small rats. I've had a few snakes that just didn't go for the small rats so they stayed on the 4 day rat pup feeding schedule longer than some of my other BP's.