Weigh your snake, and feed it a rat about 10-15% of the snakes' weight. If the snake is 400grams, go with a 40-45gram F/T rat. What you don't want is a mouser (a ball python who only eats mice). Once a week is probably the best. And feed the snake in its own enclosure to lower the stress factor of the snake and less likely for you to get bit.