You can't predict brake track wear by miles.

My tubular tire road bike is ridden on open roads and can see miles upon miles without me touching my brakes. If I hink back, I can remember 100 miles where I used the brakes fewer then 5 times total (if that). Those rims stat out only 1mm thick, and don't have much to give up, yet brake track wear is a total non issue.

OTOH- my commuter sees tons of brake use, including more hard stops in an hour than the road bike might see in a week of touring, so I see plenty of brake track wear. The last rims go to where failure would have been soon, but a T-bone crash put both out of their misery just in the nick of time (I was OK).

As a rough rule, I'd allow rims to wear about 1mm or roughly 1/2 the original thickness before worrying. But a realistic guideline, even if you measure precisely, is impossible, because a major factor is the "hoop stress" caused by tire pressure, That's proportional to the Pressure X the tire width, and so can vary tremendously rider to rider.

When I expected that my commuter's rims were near the end, I used a "safety method", pumping tires to 15-10% above riding pressure, then bled them off before riding. My theory was that if they held at 15% higher, that would be a safety margin when riding, and if all worked as intended, the failure (if it happened) would be when pumping rather than riding. Had it not been for an inattentive driver, I might have been able to report on whether my idea worked.