I've found with my snakes, the smaller (younger) ones eat more often, while the older ones will eat less often. For those less than 300 grams they've eaten every 5 days. Between 300 and 800 grams they eat every 7 days, and then beyond 800 grams they'll eat every 7-10 days. This has worked very well for me so far.
Nice of you. My 700 something gram bumblebee was eating every other week, a small rat. But now because I feel bad when I scent the room and he comes up looking for food, on his week that he's supposed to be off food I give him a rat pup. That way it's still something.