Two large rats every two weeks is plenty. Boas should remain lean and muscular. It's when they become round and fat looking that you have a problem. Of course, you should not be able to see spine or ribs, but what we consider maintenance feeding for balls is actually proper feeding for boas. With mine, I will give her an extra rat every other month or so if she appears hungry. She lets me know if her bi-weekly feeding isn't quite enough and I'll give her a booster feeding persay lol.