From wiki: "In desert areas, dust and sand storms are most commonly caused by either thunderstorm outflows, or by strong pressure gradients which cause an increase in wind velocity over a wide area."
So you can get localized dust storms from the outflow of a single thunderstorm, which typically lasts an hour or less and may affect only a single county. Arizona haboobs are frequently of this variety.
Or you can have have regional dust storms caused by gradient winds over large areas of many hundreds of square miles, covering multiple states, that can last in extreme cases for several days.