How long does it take to establish good habits?

From Oliver Burkeman’s Help! How to be slightly happier and get a bit more done:

…a new study by the University of College London Psychologist Phillipa Lally and her colleagues helps confirm. On average, her subjects, who were trying to learn new habits such as eating fruit daily or going jogging, took a depressing 66 days before reporting that the behaviour had become unchangingly automatic. Individuals ranged widely – some took 18 days, others 245 – and some habits, unsurprisingly, were harder than others to make stick…

And:

Another myth undermined by the study is the idea that when forming a new habit, you can’t miss a day or all is lost: missing a day made no difference. Indeed, believing this myth may be actively unhelpful, making it harder to restart once you fall off the wagon…