The shout will remain on you until you make the required amount of attacks. For this reason, it's beneficial to use the skill before a fight even begins. This way, you can get over the recharge time of the skill while in combat.
If used with multiple-hit skills like Barrage and Cyclone Axe, each hit counts as a separate "attack".
Despite having no listed duration, this skill does have a duration meter under it on the Effects monitor. It is estimated that the duration is 7.5 hours.