How Long Does It Take to Develop a Habit?

There have been anecdotal accounts of how long it takes to develop a habit. Habits are cognitive associations, their mental associations that we form when we repeat an action over and over again in a given context. Some habits, in some ways, are easier to form than others. Perhaps they have larger rewards than others. But also for complex behaviors, for things that have many different parts, it takes longer to form a habit....