You should be feeding your snake a rat that is approx. as big around as the widest part of your snake's body. I feed all of my ball pythons every 7 days. Every snake will be different, of course, but my recommendation is to start there and adjust as needed. This is basically the same as the feeding guidelines already posted, but if you can't measure in grams you can at least measure by visually comparing size. It doesn't have to be perfect - some BPs will even take prey that is slightly larger, some prefer a smaller prey item. Or you could get really lucky like me and have one that likes a large rat one one week and a medium one the next. (She's very picky.)