The answers to this question depends, in large measure, on the problems that you choose to address in your therapy. 

Some problems respond very well to a short course of therapy, while others require a longer commitment. 

In general, a more narrowly defined issue, in which the desired outcome can be concretely named (e.g. “I want help in confronting my boss about a raise. I would like to be able to handle the interaction professionally, but in the past I have always ended up getting angry or teary”), may be resolved in a few sessions.

Conversely, problems that are more generalized (e.g. “I’m unhappy but I can’t figure out why, and I don’t know what would make me feel better.”), would probably require a longer discernment process to get to the real issues that are causing the distress. Additionally, these kinds of issues take more time to effect the changes that would be necessary to resolve the problem. 

The scope of the therapeutic endeavor can be negotiated between you and your therapist so that you know what you are committing to, and what the criterion would be for a successful outcome.