The old belief was that it took 21 days to change a habit. This has now been labeled a myth - it takes far longer to break an existing habit. Evidence is building to support 90 days as the point at which the brain can reset itself.
Researchers at Yale University have documented what they call the sleeper effect–a gradual re-engaging of proper decision making and analytical functions in the brain’s prefrontal cortex–after an addict has abstained for at least 90 days.
https://time.com/archive/6681513/how-we-get-addicted/