We have only one guy at our field who has been instructing. Since he doesn't get to fly his own stuff during the weekends, I volunteered to take a bit of the load off of him. He was glad I did. I have a couple of guys that I am helping now and I don't have any criteria for when they are going to be ready. I think it will just depend on if I have to let go of the button too many times, you know what I mean? I think having a list gets a little out of hand. I think most instructors can tell when a person is ready to be cut loose or not. If your thumb gets sore, then let 'em go.