Lally et al. 2010, how habits form in the real world

European Journal of Social Psychology field study of 96 people adopting a new daily eating, drinking or activity behaviour in a consistent context. Self-rated automaticity rose along a plateauing curve rather than a straight line as the behaviour was repeated. The median time to reach the automaticity plateau was about 66 days, but individuals ranged from roughly 18 to 254 days, so any single “days to form a habit” figure is highly variable. Repeating the behaviour in a consistent cue or context was what built automaticity, and missing an occasional day did not meaningfully set the process back. A single, well-cited observational study using self-report; foundational but modest, and the 66 days is a median with a wide range, not a rule.