Mastery scores help students track their progress toward becoming an expert at any skill. Think of mastery scores as a progress bar, rather than a percentage correct, tracing students' most recent progress and growth. A mastery score of 88, for example, suggests that a student is 88% of the way toward the goal, not that the student has answered 88% of questions correctly.
Also note that mastery practice is adaptive, such that questions get harder or easier based on a student's performance. Because of this, students will not all see the same questions and some students may need more or less time to master a single skill. Generally, we estimate that it will take 5-10 minutes to master each skill on the site.