This is a question that clients often ask at the start of counselling. Unfortunately there is no easy answer as it depends on a number of factors. The most important of which is the relationship that you have with your counsellor. You will need to allow time to develop trust with them and to feel safe enough to talk about your vulnerabilities. Therapy can be painful and uncomfortable but that's not to say there aren't joyous and liberating moments too!
Your counsellor will also want to feel comfortable enough in the relationship to be able to challenge your deeply held beliefs and offer you a different perspective. If your relationship is strong then it will withstand this.
Many counsellors suggest six weekly sessions initially, to establish a trusting therapeutic relationship. This can be very helpful and I have definitely seen the biggest change when people start out in this way. For some people, six sessions may be all they need. For others this may be just the start of the journey and they go on to have counselling for months or years. Some people will dip in and out of counselling throughout their lives to address issues such as trauma or mental ill health or purely as a form of self care, to prevent burn-out or emotional fatigue.
It is important to be patient with yourself when you consider the length of your counselling journey. You may want to change patterns of your behaviour that have become ingrained over a lifetime. That won't happen overnight but the results could be life-changing and well worth the wait.