Samurai Limit Break: Meikyo Shisui

[Azurift]

Meikyo Shisui (Su): This Limit Break grants a +2 bonus to all attack and damage rolls made by the samurai while wielding a katana. This bonus increases by 2 every four samurai levels after 1st. Furthermore, he gains a point of Sen each round he spends in this Limit Break. If he drops a foe below 0 Hit Points while under the effects of this Limit Break, he gains a point of Sen. He can only gain a point of Sen this way once per round. This limit break requires only a swift action.

This does not state anywhere how long this is supposed to last. Is this intentional?