You don't<em>have</em>to add a duration as such. What the duration is for is to have the attack<em>continuously going off for that amount of time</em>(i.e. an incendiary grenade that lights the ground on fire for X amount of time). Since the Entangled status is<em>already</em>persistent(until destroyed) the duration would simply hit them with the attack again (and thus reapply entanglement) for that long.<br/><br/>So basically it would be good for making it much harder to escape (since they'd have to break free of the entanglement<em>and</em>move out of the radius in a single turn) but isn't needed to<em>keep</em>them bound<em>unless</em>they are actively trying to escape.