How thick should the smoke be? It should be almost invisible, if you can smell it that's good.
Should the box be full? If you are using chips adding a small amount each time is the best.
Should it be thin smoke where you should still be able to see the meat or thick where it makes it hard to see? You do not want billowing white smoke, if you have the window in the door, the meat should be clearly visible, if it's too smoky to see the meat, it's too much smoke. Make sure the top vent is open all the way.
Should you wait until the smoke stops to add more chips? Yes
Should you clean out old chips each time right before you add more chips? No, on a long smoke like pork butt, you may need to clean it out periodically, especially if you have the small chip box, but it's not necessary each time.