How long can it take to change a habit?

I read an article from the psychologist dr Philippa van Jaarsveld who says that it takes an average of 66 days to create a new habit. A simple habit like starting to drink a glass of water every day can take up to 18 days to get into as a habit. A more difficult habit like doing 40 sit-ups every day before breakfast can take a full 254 days to get into as a habit. We humans like to stay in our comfort zone and continue to live as we always did instead of changing habits that are often both strenuous and uncomfortable. It is easy to choose short-term rewards before one that is further ahead. It is easier to stay in the comfort zone instead of taking the more strenuous path of getting the long-term reward.                                    For example, if I want to lose weight, then I decide to start going to the gym. The reward, the weight loss, will take time and requires a high degree of discipline. The short-term reward of skipping the gym and instead enjoying watching a film with a glas of wine tends to be much nicer. We often say to ourselves that I will do my training tomorrow instead.