When To Step It Up

[AngelSnake]

I just got my snake last Thursday. She/He (the owner was unsure so I just go with she until shes bigger and I am not afraid to hurt her while checking) is fed every Wednesday so I just fed her for the first time this morning. She is a very eager eater so no worries there. She's a king snake and around 10-11 inches long and about the width of a pencil. Im not sure how old that would put her at being fed pinkies once a week.
Anyways getting to my main question other than age. She still seemed hungry after her pinky this morning. When do I start feeding her two or even fuzzies? I was told meals twice the size of her head, but Im not sure how reliable that is considering the owner didnt even know her sex.
Thanks from Diamond & Cheryl