When people have nurturing, protective, wise parents and no trauma throughout childhood, therapy is likely to be short. Usually in this case, people come to therapy for an event such as a rape, abortion, mugging, or car accident; a loss such as a death, job loss, or divorce; or a dilemma, like being unhappy in a relationship or job.
In these cases, just talking about the issues, grieving the losses, getting compassionate understanding and wise insight, can resolve the situation. Depending on how much and how deeply you want to make use of therapy, this can be one session, or six months.
When people have been repeatedly traumatized, abused, neglected, or shamed as a child, without loving adults to help them handle these traumas, they generally need a longer time in therapy, When people get hurt in relationships that are supposed to be close and trustworthy (like parents), it takes another committed and consistently trustworthy person to help repair those wounds over time. When people have been badly hurt, especially when they were children, repair is generally a slower process of developing trust in the therapist and transforming childhood ways of coping into more effective ways.
I encourage you to attend the first session with the idea that you will be there for 3 or 4 times, and as the trust builds and barriers come down, who knows where you will allow your therapy time to reach.