In my opinion that is when you are close to the end. It becomes more of a hassle to deal with the effects of his drug abuse then to actually stay in the relationship. Welcome these feelings they will help make your decision to leave that much easier.
I was just talking to a friend about all the crap that happened leading up to me asking my ex to leave. Brings back the whole feeling of hopelessness you feel while watching them continue to do this to themselves.
Maybe you are detaching by being numb. Making yourself indifferent. Self preservation.