How Many Days Does It Take to Change a Habit? (According to the Research) How long does it take to change a habit? You have probably heard 21 days, or maybe 30 or 40? In a recent study by University College London, they discovered that it takes an average of 66 days to change a habit like eating fruit at lunch. They also found, if you miss a day here or there, it doesn’t affect your results, but being diligent at the beginning has more impact.