Perhaps the best answer is, when people quit asking the question. I do not believe that standardized test results are an adequate measure. I do not believe that the impressions of college mathematics professors are an adequate measure. High school mathematics will be considered successful when all partners in the education process collaborate to assure that students are prepared for a technical world fraught with unknown circumstances and unplanned events. This requires a commitment from society at large to support education in a variety of ways.
Family support is the initial condition.
Adequate resources to support a quality education is next. Job opportunities for successful students must be available. The work of mathematics and mathematics-related careers must be understood and valued by society. The role of mathematics in being a productive citizen and understanding things like voting and budgeting must be advanced. When these factors are present, students will have motivation to be successful mathematics students.