Maybe I have just been lucky but in the 14 cruises that I have been on I have never smelled any left over smoke in my cabin. I am a non-smoker.
NCL allows smoking in the cabins, on the balconies, in the casino, and in some outside areas (port or starboard) by the pool and outside decks. There is also smoking allowed in the smoking/cigar lounge of course. I really have not been bothered by smoke on NCL ships except in the casino. I found it difficult to be in there on one cruise. I was thankful for this since it saved me some money!
I remember the days when smoking was allowed in the dining rooms and in all lounges.