DBs aren't time-limited for GMS.

So it's official that DB's skillbooks will be CS-exclusive?

>Certain Dual Blade Mastery Books will be available in the Cash Shop but it is not required make or level up a Dual Blade character. If you are interested in purchasing the Mastery Books, you can check out our earn Free NX offers to pay for the books.
>> Yes, and they will be in the CS. But don't worry, it is not required that you level up a DB character! If you want to, though, you can check out our surveys.

Wow that's disheartening. It's like a polite way of saying "Screw off if you don't have NX." =/