I can think of two ways to interpret that, both falsifiable: 1. people are mistaken about their ability to concentrate for many hours. This is easily falsified simply by looking for some objective measure of concentration. EEG frequencies, bug rates of checked-in code, performance on any of a thousand psychological tasks/tests, etc. We could refine this with explanations for why they are mistaken about their endurance: because their cognitive abilities deteriorate over time including self-monitoring cognition? Self-deception, possibly related to status-seeking? Either of these could lead to a lack of knowing what it really means to concentrate. 2. people do not put forth peak effort but average effort, and they can do average effort for many more hours and have accurate beliefs, but they are mistaken when they try to distinguish between peak and average. Perhaps it has been too long since they exerted peak mental effort and they've forgotten how unchallenging their daily activities are (I think of intelligent people who crack a math text and remark how much their head hurts and how tired they are after a few minutes). The difference can be gauged by lengthy testing with and without high stakes or other forms of pressure, and then one simply watches performance over time: if someone performs better under high stakes and their performance does not drop off over many hours, then their claim was right. (Concrete example: subjects take 3 back-to-back SATs covering ~8 hours, for no stakes; then the next day a gun is put to their head... Does SAT 3 have the same or higher score as SAT 1 under both conditions?)