What metric do you use to estimate remaining brake pad life on a typical economy sedan?

Not a cop out just a fact. I know of people who do burn through front pads in under 10K and I know of others with 60K on fronts. They drive differently and use different parts. The only time you can make a guess is on your own vehicle, because you know how you drive.

There really is no average when it comes to brakes unless you know how that person drives, where they drive and the parts they used. I never gave a guess on pad life, I would however measure the pads and tell them what was left. If it was a new car then you have a mileage to use, but even that wouldn't matter much if the car had been setting for a while and now it's driven 6-800 miles a day.

I've gone through pads in 10,000 miles. The rule of thumb you seek depends on driving conditions.

When you first brought this up I mentioned my experience from city living to rural living the the difference is huge. AVERAGE is meaningless unless you know where you stand on the parameters. .

From that, I can conclude you don't use you car as a taxi in NYC or Chicago.