As you're drafting your New Year's resolutions, you may think that it takes 21 days of repeating an action for that action to become a habit. So, you set out to go to the gym for 21 days, thinking that by day 22 heading to the gym will feel automatic—maybe even fun.

However, that 21-day estimate isn't true. A habit expert Wendy Wood found that this falsehood came from a self-help book in the 1960s and actually described how long it takes to get used to your new appearance after plastic surgery. It's important to note that getting used to something is not the same thing as forming a habit, and perhaps the two concepts got confused along the way, giving rise to the term today.

To figure out how long it actually takes to form a habit, Phillippa Lally, and her team did a series of research.

They recruited 96 people and asked each person to choose one new habit and report each day on whether or not they did the behavior and how automatic the behavior felt. Some people chose simple habits like "drinking a bottle of water with lunch." Others chose more difficult tasks like "running for 15 minutes before dinner." At the end of 12 weeks, the researchers analyzed the data to determine how long it took each person to go from starting a new behavior to automatically doing it.

The study found that it takes more than 2 months before a new behavior becomes automatic—66 days to be exact. And if you want to set your expectations appropriately, the truth is that it will probably take you anywhere from two months to eight months to build a new behavior into your life. Interestingly, the researchers also found that "missing one opportunity to perform the behavior did not affect the habit formation process." That is to say, it doesn't matter if you mess up every now and then. Building better habits is not an all-or-nothing process.

So, for those who wish to form certain habits, persistence is king. To form a good habit, we need to pay more energy and effort.