Air filter restriction is not time or miles dependent.

Mileage of as much as 300,000 to as little as 5,000 miles have been reported when restriction is reached.

It is important to know the highest full load restriction that occurred during the vehicle operation in order to service BEFORE the redline.  

After an element has reached its redline value, it only takes a little more contaminant before high fuel consumption and poor engine operation occurs.  Don't Dust Your Engine!