The honest answer is that it depends, which is useless on its own. So here is what it depends on, which is not.
The first checkpoint is six to eight sessions
Not for resolution. For evidence that the method and the person are right for you. By session eight you should be able to point at something — sleeping differently, catching a state before it takes you, a conversation that went differently than it would have in March.
It does not have to be big. It does have to be outside the room. Feeling good during sessions is not evidence; therapists are pleasant company and an hour of undivided attention feels nice regardless of whether anything is changing.
What lengthens it
Developmental rather than event trauma. If what you carry is a climate rather than an incident, there is no single thing to process — you are changing a default, and defaults change by repetition. That is months to years, not weeks.
A narrow starting window. If you arrive with very little tolerance for your own internal state, the early work is building the capacity to do the work, and that is real time that can feel like nothing is happening.
And ongoing conditions. If you go home to the situation that produced the problem, therapy is swimming upstream. That is not a reason to stop. It is a reason to expect a different pace and to be honest about what is being asked of the hour.
What shortens it
A single discrete event with a life that was otherwise stable. Sleep that is basically intact. Someone in your life you can be honest with. Having done therapy before, so the first month is not spent learning how to be in it.
A rough shape
Single-incident work with a settled system: often three to six months. Developmental or complex material: a year or more, frequently with stretches that plateau. Maintenance afterwards, if you want it: monthly or as needed, which is a legitimate way to use a therapist and not a sign of dependence.
Ending it
You are allowed to stop. You do not need a reason your therapist agrees with, and a good one will help you leave well rather than treat the wish to go as material to analyse.
Most people stop too late rather than too early, because ending is awkward and the appointment is already in the calendar. If you have not been able to name what you are working on for two months, that is the conversation, not another session.