Dust of Twilight - duration of fatigue?

[Tusk the Half-Orc]

Dust of Twilight is a level 2 bard, bloodrager, and sorcerer/wizard spell. It has a duration of "instantaneous," covers a 10-foot spread, and has the following description:

Dust of Twilight wrote:

A shower of iridescent black particles clings to and extinguishes torches, lanterns, sunrods, and similar mundane light sources and dispels any spell of 2nd level or lower with the light descriptor (as dispel magic). Creatures in the area must make a Fortitude save or become fatigued.

What is the duration of the fatigued condition? Instantaneous doesn't really make sense for this part of the spell effect.

[Firebug]

Fatigued until you clear the condition, typically by sleeping. Waves of Fatigue is the same.

Spells that have limited duration on fatigue, like Touch of Fatigue say so in the description.

[Tusk the Half-Orc]

Excellent, thanks!