Interesting question. It is my opinion that it comes with experience and confidence in ones own judgement and ability.
I recall feeling as you did but I also recall it wasn't for very long.
The fact that you are concerned about intervening too much is probably an indication that you have the right attitude in wanting to give your best and not rob the student of necessary practice.
My bet is that after a few hours you will look back and wonder what you were worrying about.
Talk to another instructor or your CFI if he/she is approachable. We have all been there once and they should be able to reassure you that your concerns are quite normal.