To change a bad habit (which would include negative self talk) takes 21 weeks... and that is if you replace it with a good habit (such as positive self talk.)

We have an advantage though when dealing with self talk and thoughts. It takes 30 seconds for a thought to become fixed in the brain. The brain can only think one thought at a time. That means if you say something negative (or think it) you have 30 seconds to make it into a positive thought ...which said positive thought would become fixed or stabilized after 30 seconds.

If you don't dwell upon the positive thought for 30 seconds, but move on to another thought, and maybe even a negative one (such as, I said that but I don't believe it) then you don't reap the benefit of the positive thought.