Keep digging until you can see light coming through the snow of the walls. Snow is translucent enough that even when it is about a foot thick, you'll be able to see sunlight coming through. A thickness of one foot is good for the walls & ceiling. If you want to be more precise about it, you can poke foot-long twigs into the snow pile before you begin to hollow it out. Then, when you dig up to the twigs, you know the quinzee's walls are as thick as you want them to be.