I offer my 07 Calfornia king at least one pinkie a week. Quite often she gets two, if one of my pyros refuses a meal. She is really big enough for fuzzies, but I only have pinkies in stock right now. So to answer your question, a mouse about as big around as the widest girth of the snake about once a week should do fine. Don't feed live mice that can bite back, though! Snakes are in general rather lethargic creatures, no match for a feisty mouse!
The pet store just got a california king snake in. She is 15 inches and very very sweet, but not a morning reptile. My other animals wake up at six thirty and she will have to wake up at seven. I named her oreo because she looks like a bunch of oreos stacked in a line.