1) I generally place no restrictions since all of the solos at our flight school are supervised. i.e. I am there with the student every time. We discuss weather and the planned flight each time and make a decision together.
2) It took 200 hours of instruction to be comfortable teaching. (400 TT)
I was proficient at flying autos myself when I became an instructor but until I knew what to expect from the student while in an auto my concentration could only be focused on keeping us in a safe profile, not on teaching.
3) A good student is one who wants the information and gives feedback. If one of my students shows up before a lesson with a multitude of questions I know they've been thinking helicopters during the time since we last met. If someone asks me to expand on part of a definition or procedure (in the air or on the ground) I know they are absorbing the information.
Some words of advice to readers: Know your course. Depending on which course you are teaching you need to know which maneuvers and subjects are required for that course and what the standards are. This will promote primacy and lessen over-teaching.
Fish