Hypnosis works by communicating with the unconscious mind. Whereas the conscious mind controls things within your awareness, such as moving your hand or making a phone call, the unconscious mind controls all of the autonomic processes that you don't have to think about, such as heart rate and blood pressure. It's also where our thoughts, memories, and beliefs reside. In any given second, the unconscious mind processes about two million pieces of information. So although you aren't consciously aware of all that activity, your behavior is profoundly influenced by it. Hypnosis helps you shape the beliefs in your subconscious mind so that you behave differently in the future with very little effort. As you can imagine, we all have a lot of subconscious beliefs that can influence our choices and behaviors. The key is understanding that the belief isn't a fact, it's just a subconscious belief and it can be changed! Hypnosis works by communicating directly with your subconscious mind to change its outdated, limiting beliefs.
Highly hypnotizable people may experience benefit in as little as one session. Some people are either unable to be hypnotized or have low hypnotizability, in which case self-hypnosis may not be effective, or it may require listening over a longer period of time to notice changes.