Snakes are very individual. There is no standard weight for a rat snake of any particular age. A rat snake that young may need to be fed more often than weekly, though, depending on the size of the meals. With my corn I see if the lump from the meal is gone by the third day I will feed another meal 5 days from the previous meal. Also if the lump is gone in 3 days or less it might be time to move up in food size.