The length of counselling varies according to your goals for counselling, your life circumstances, and your life history.
In simple cases, counselling can take only around 5 sessions. An example of a simple case would be an adult who grew up in a healthy home environment, is able to take good care of him or herself on a day-to-day basis, and is seeking treatment for the resolution of PTSD related to a recent motor vehicle accident.
In complex cases, counselling can take several months or even years. An example of a complex case would be someone who was abused and/or neglected when they were growing up, currently lives in an unstable environment, and is struggling with addiction. This person can still find healing and happiness - it just takes longer and harder work.
Most people are somewhere in the middle between simple and complex.
After completing an assessment with you, your therapist should be able to give you an idea of how long counselling will take in your specific case.