I have a 6 foot boa, and she eats one medium rat or two small rats every two weeks. A couple weeks ago, I tried to give her a rat after only a week and she refused him. Your boa should be absolutely fine on one every other week, but if you notice that she immediately goes for the rat every time and still seems to be looking for more when she finishes, then you could always try a second. I use a separate feeding box for my boa, and rather than reaching in when she finishes eating, I use a yard stick to pet her with to let her know that there's no more coming. If she strikes the yardstick a couple times, I realize she might still be hungry.