It took me a few days to become a smoker, and a few weeks to get to the point of not wanting to smoke again (some decades later).<p>It took me a week or so to start having a healthier breakfast and it become part of my routine, and I've been trying to have healthier lunches for years now (and still regularly fail, normally because at lunchtimes I'm tired, stressed and yearning for comfort foods like sandwiches and snack bars).<p>I think anyone who believed exactly 21 days was the magic number for all habits, for all people, was grossly naive.<p>But what I do find is that after 21 days it's no longer novel, it's just what you do, if somebody asks you what you do about X, you no longer say "I'm trying this new thing around X, and...", you tell them your new habit. Identifying that habit as part of who you are is key to it being sticky. For some people, and for some habits, that might be true after a week, or it might take a year, but it's an important step, and if you want that habit to stick you should get to it as fast as you can.