The Haboob can continue for a number of hours. Khartoum will issue sigmets when viz is expected to be below 1000m.
If the viz is reported to be below 200m then diverting to your alternate is the only option. If you're flying over from Europe you an skip over the pond to Jeddah and hang about unil it blows itself out, usually by midnight it would've cleared up nicely.
The famed Haboob is caused by approaching Thunderstorms, which usually hang around for 2hrs max before the area is clear again.