I believe that you should stop when something has nothing good to offer society. There is really no use for trash to be sold on video, in books, etc. It does us no good.
Age appropriateness is also very important.
As I am learning in psychology, the human mind is a very fragile thing. I think it is a good thing that there are restrictions on certain thing. I think TV is one of most commonly used objects that allows children access to things that aren't appropriate for them. That is where parental guidance and supervision come in.