I believe db/(pre-)re/skill_cast_db.txt is what you're looking for.
// Structure of Database:// SkillID,CastingTime,AfterCastActDelay,AfterCastWalkDelay,Duration1,Duration2,Cool Down//-- SL_KAAHI463,0,500,0,1800000,500,0
Change 1800000 to 30000, I think it should do the trick.