On cars we service at the suggested service intervals of 7,500 or 10,000 miles, we can estimate the remaining mileage quite well. As far as replacing the brake discs, there is a 'wear limit' for brake disc thickness particular to the particular vehicle. If your discs are worn to that point, they must be replaced. IF they are not worn to that limit, we would resurface the discs, IF they will still be above the service limit after resurfacing.