What is the duration of the resistance effect of Swarming Dispersal for a Swarmkeeper Ranger?

Question

In the Ranger section of Tasha's Cauldron of Everything appears a new version of the sub-class Swarmkeeper, which is quite distinct from the UA version.

There is a new ability called Swarming Dispersal (TCoE, p.60). The feature does have an effect for which no duration is specified:

Swarming Dispersal

15th-level Swarmkeeper feature

You discorporate into your swarm, avoiding danger. When you take damage, you can use your reaction to give yourself resistance to that damage.

You vanish into your swarm and then teleport to an unoccupied space that you can see within 30 feet of you, where you reappear with the swarm.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Is this an instantaneous effect, does it last until your next turn, or does it have another duration?

I'm looking for a RAW answer, which potentially includes a comparison with examples of similar features in other classes.

Answer 1

Resistance against an instance of damage, not a type

The feature refers to that damage meaning that the damage of the triggering event is halved by the resistance given. This means that the feature does not have any duration at all. We can call it instantaneous.

Since the qualifying property is that the damage comes from the triggering event - a condition which can never be fulfilled again - it is unnecessary to specify any duration.