use of a dash

Hi,

I am not too sure about the punctuation for certain dash use cases.

Case involving a relative clause: aw He spend hours day and night writing newspaper articles -- and, of course, not going out with his friends as he ususally did on weekends -- but he couldn't finish it on time. Sad case, indeed.

Case involving a long content: Study is the priority here, and it is so in every school -- it's the priority in our personal life, that we be men and women of learning whom parents have called us to be.
Q: How long a content should the content after a dash be??

Case involving a subordinate clause: Although he may lack in discipline -- and many people think so too -- he continued to work on the task of writing the book.