While the Acuity node for Arcane Ascendance grants it a duration of 12 seconds & max number of spells cast, the node after (Cunning) grants it more spells and a longer cooldown. The stats listed under this description, though, seem to imply it also modifies its duration. See below.
This is a bug as the skill does not last longer than 12 seconds regardless of the points allocated in Cunning. I do not know whether the skill is intended to last longer with more points allocated in Cunning or if Cunning is not intended to modify the duration of Arcane Ascendance.