Man, it depends on so many things.
In addition to what
@Tricia said, it definitely depends on the snow quality. I've had deep days that were anything but "good powder days". I've learned to keep my expectations in check until I see how the snow came in, both in terms of moisture content and what the wind did to it as/after it fell. And sometimes, finding the right aspects on the mountain is what makes the difference between a great day and an exhausting day dealing with dense funky snow. And sometimes, it depends on what's underneath that new snow. I've had 6" of fresh ski *way* better than a foot, because that foot is on top of a layer of refrozen crud.
I could give you a number, and then also rattle of tales of days we got that number and it still wasn't a "good powder day", and vice versa.