There is no easy answer to this question. I think the best answer is that it depends on what brings you to therapy, who you are and what you want to achieve.
For example, if you have come to therapy because you are in a job or relationship that makes you unhappy, are fairly sure you want to leave the situation and just need to talk it through and get it clear in your own mind, in the confidential non-judgmental therapeutic setting, you may find resolution very quickly; maybe in just a few sessions. Using the same example but in the case that therapist and client identify a pattern such as a string of relationships and jobs that go the same way then it is likely that you are the common factor and longer term work will be required to change the pattern of difficult circumstances. In other words, it's your personality that either gets you into the kinds of jobs, friendships or intimate relationships that don't work for you or that makes you struggle when jobs and relationships begin to age.
Counseling / psychotherapy also don't have to only be about solving problems. Therapy can also be a hobby or used on an ongoing basis to improve effectiveness in work and personal relationships. Some people never stop having therapy – I had personal therapy for around twelve years.
Regardless of your motivation to see a counselor / psychotherapist, in my experience it is possible to gain some insight into a problem and change the way you think even in the first session while major changes in personality and levels of contentment seem to take around three to six months. This is based on my experience and observation and though it may be backed up in research, here it is my opinion only.