There are various research articles done in the past which describe rates of change in psychotherapy patients, some studies for example describing a plateau after about 25 sessions or so. I find these studies very weak, because of the multitude of confounding factors: severity and chronicity are obvious variables, also the type of follow-up assessments done.
In the CBT literature, a typical trial of therapy is perhaps 16-20 sessions.
In light of our evolving knowledge of neuroplasticity, and our breadth of understanding about education & learning, it seems to me that the most important variable of all is the amount of focused, deliberate practice time spent in a therapeutic activity. Oddly, most psychotherapy studies--even CBT studies--do not look at how many hours of practice patients have done in-between therapy appointments. This would be like looking at the progress of music students based on how many lessons they get, without taking into account how much they practice during the week.
I have often compared psychological symptom change to the changes which occur, for example, with language learning or with learning a musical instrument.
So, I believe that a reasonable estimate of the amount of time required in psychotherapy depends on what one is trying to accomplish:
-Some types of therapeutic problems might be resolved with a few hours of work, or with a single feedback session with a therapist. This would be akin to a musician with some kind of technical problem who needs just some clear instruction about a few techniques or exercises to practice. Or it might be akin to a person who is already fluent in a foreign language, but needs a few tips from a local speaker about idioms, or perhaps some help with editing or grammar in a written text.
-Many more therapeutic problems could improve with perhaps 100 hours of work. This would be like learning to swim or skate competently if you have never done these activities before. Regular lessons ("therapy") would most likely speed up your rate of progress substantially. But most of those 100 hours would be practice on your own, unless you're okay with the progress taking place over a year or more. With the language analogy, think of how fluent you might become in a foreign language with 100 hours of focused, deliberate practice. For most of us, this would lead to an ability to have a very simple conversational exchange, perhaps to get around in the most basic way in another country.
-A much larger change is possible with 1000 hours of work: with music, one could become quite fluent but probably not an expert. With a foreign language, comfortable fluency would probably be possible, though probably still with an accent and a preference for the old language.
-With 5000-10000 hours of work (this is several hours per day over a decade or more) one could become an expert at a skill or a language in most cases.
In psychotherapy, another confound though is whether the times in-between "practice sessions" lead to a regression of learning. An educational analogy would be of practicing math exercises an hour per day with a good teacher, but then practicing another 8 hours a day with another teacher whose methods contradict the first. Often times, learning will still take place with this paradigm, but it might be much less efficient. Persistent mental habits, in the context of mental illnesses, can be akin to the "second teacher" in this metaphor, and unfortunately they do tend to plague people for many hours per day.
This reminds me of the evolving evidence about stroke rehabilitation & neuroplasticity: substantial brain change can happen in as short a time as 16 days--but it requires very strict inhibition or constraint of the pathways which obstruct rehabilitation. (note: 16 days of continuous "immersion" = 16*24 = 384 hours!) In stroke rehabilitation, the neuroplasticity effect is much more pronounced if the unaffected limb is restrained, compelling the brain to optimize improvement in function of the afflicted limb. Here is a recent reference showing rapid brain changes following limb immobilization: http://www.ncbi.nlm.nih.gov/pubmed/22249495
In conclusion, I believe that it is important to have a clear idea about how much time and deliberate, focused effort are needed to change psychological symptoms or problems through therapeutic activities. A little bit of meaningful change could happen with just a few hours of work. In most cases, 100 hours is needed simply to get started with a new skill. 1000 hours is needed to become fluent. And 5000-10000 hours is needed to master something. These times would be much longer still if the periods between practice sessions are regressive. In the case of addictions, eating disorders, self-harm, or OCD, for example, relapses or even fantasies about relapse will substantially prolong the time it takes for any therapeutic effort to help. Of course, it is the nature of these problems to have relapses, or fantasies about relapse--so one should let go of the temptation to feel guilty if there are relapses. But if one is struggling with an addictive problem of this sort, it may help to remind oneself that the brain can change very substantially if one can hold onto to quite a strict behavioural pattern for the hundreds or thousands of hours which are needed.
As a visual reminder of this process, start with an empty transparent bottle, which can hold 250-500 mLof liquid (1-2 cups), and which can be tightly sealed with a small cap. Add one drop of water every time you invest one hour of focused, deliberate therapeutic work. The amount of time you need to spend in therapy depends on your goal. If the goal is total mastery--then you must fill the entire bottle. If simple competence in a new skill is an adequate goal, then you must fill just the cap of the bottle. If there are activities in your day which contradict the therapeutic work, it would be like a little bit of water leaking out of your bottle. So you must also attend to repairing any "leaks." But every hour of your effort counts towards your growth.