Since it has been many years since I have taken a cruise of less than 10 days, I can't personally speak as to the level of compliance on a 10 day (or longer) cruise versus a shorter cruise. I can say that I have seen plenty of people (including myself) entering the dining room and showroom in dressy casualwear on formal nights and I have yet to see anyone turned away.
A previous poster posted that you won't see casualwear being admitted on 10-day and longer cruises. I can personally attest to the fact that you will. In fact, I can say that I have noticed more non-compliance with the dress code on my most recent 10-day cruises than I did on my 7-day cruises of just 5 years ago. I'm sure that has more to do with the passage of time than the length of the cruise. Society is becoming more casual and I doubt that is going to change anytime soon.