This is terrible advice. Box squats should most certainly be used for ME work. The only reasons you wouldn't get anything out of them is because you use the same box height for too long (that was probably too high) and your form is terrible (getting too loose on the box, not sitting back far enough, not chainging foot positions, not driving knees out, etc.) If all of those things were in check, then I have no idea what to say. I have personally seen box squats make hundreds of athletes stronger, faster, and able to showcase such things on the field.
Anyway, to answer the OP, the answer is every height. Change it often. As often as you like. I would suggest not sticking with a height for longer than 2 weeks. I'm not saying have a million different heights you use but try to have 3:
Foot position used depends on your own personal weaknesses but change that up often as well. Again, just have 3:
Medium (which will probably be your regualr squat stance)
Try to close the gap between the weight used with whatever height and foot position you are strongest at (it will probably be wide stance and high box) and the one you suck at the most (probably close stance and low box).