From Dual Blade Q&A (http://forum.nexon.net/MapleStory/forums/thread/4998633.aspx)
Q: Is the Dual Blade class only available for a limited time?
A: No, Maplers will be able to make Dual Blade characters forever!
I guess we won’t have the same thing as kMS. We will have dual blade forever. It doesn’t really affect most people, but now if you mess up your build or your skills, you don’t have to worry, because you can always make a new character.
Also, it is comfirmed we are getting a free character slot.
Q: Will players be given an extra character slot to make a Dual Blade?
A: The maximum number of character slots will not be increased so Maplers that have not maxed out their character slots will receive an new character slot.