How long does a dust mite allergic reaction last?

It depends...: On the amount and duration of exposure. If you're allergic to dust mites and you fluff up a pillow on the way to the washing machine, probably 30 minutes. If you sleep on a pillow without an allergy cover, the all-night-long exposure might last a couple of hours, or if you're highly allergic to dust mites, it might last all day.

Chronic on-going: Dust mites are very prevalent indoor allergens causing rather constant exposure for many people. The allergic reaction to a single acute exposure to an allergen may last up to 48 hours, however, people generally have a more constant exposure to dust mites and reactions become on-going.