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You can see it by running a cable in a straight line from the buildings. The cable will get dusty faster in the dust zone. So you can test it if you like even though I am about to tell you its 5 hexes and it is highlighted while you are placing the building.
 
It is 5 hexes and it is also shown as a grid when you place ANY building including the extractor.

Basically, if you are trying to build anything and there is an extractor tile within the grid shown the building will get dusted.

I've found out that using rock piles is a great way to seperate extractors from the other stuff. Rock pile + pipe/cable = 5 hexes