Though depending on how seriously we take Bloom's two sigma problem (I'm never sure how much we should) the opposite may be true: A student should be able to 100% a test before they are ready to move on to the next material. Maybe we just take too long to get them to that point.
One concern I always have with the mastery approach is that I find that I understand a subject much better once I've learned the next thing, so maybe the ideal path is something more like "You should always be able to 100% the subjects you learned six months ago" or something.
