I just "graduated" from DBT about a month ago. It teaches a lot of coping skills, adn can really work, as long as you are willing to put the work into it, and practice it.
I was in it, this time for close to 18 monhts (a previous time, prior to having ect i was in for about 2 years).
How long you attend, at least where i went, was really based on how much you are learning and applying the skills