i have smogged a couple cars in the last year.. 97 honda prelude, 03 crown vic police intercepter and helped a friend smog a 300zx. The smog tech had me drive around town and back which was roughly 10-15 miles. He had me give it some heavy acceleration and slow cruising, and that reset the codes. Subarus may be different when it comes to mileage, but i would say 20 would be a good number to try if you were crunched for time.
so we are guessing now?