its not so much "miles" before replacement, as it is "when the pads get low".
for example: if i drive 30,000 miles per year, but its all highway, my brake pads might last quite some time compared to someone who drives 10,000 miles per year, but its all country roads with a ton of stop and go.
i will typically check my brake pads in march and october/november when i change out the snow tires. then plan for replacements when i notice them getting low.