"Harsh" driving conditions refers to mostly highway driving. Notice that there is a subscript (1) that explains things a bit further under Remarks:
*1. If the vehicle is operated in very dusty or sandy areas, clean and if necessary, replace the air filter more often than the recommended intervals.
So if the car is being driven on the I95 between Miami, FL and Houlton, ME for most of its life, change the filter at 30,000 miles. If it is a mix of the I95 and some city driving in a few major cities along the way, 22,500 miles. But if the car spends its time in the deserts around Albuquerque, NM then you will need to change the air filter at a more frequent interval. Rule of thumb, generally, is to remove the filter and hold it up the the sun/light source. If you can see the light source through the filter, you can continue using it. Significant blockage of that light source means its time to change it.