How Long Does Treatment for Depression Take?

There are many variations of "depression," including a range of severity from mild to severe, as well as whether it involves mood swings which intensify it, whether it involves anxiety as well, whether it involves specific triggers such as a death or loss of a relationship, and whether there are ongoing sources of triggering in the individual's life such as a problematic relationship. All of these factors will affect the length of time it may take to reduce the severity of the symptoms.
Of course, the willingness of the patient to take suggestions, do the "homework" in between sessions, and to consider and reflect on the conversations we have, will also affect how quickly the psychotherapy works. I often say that depression is like, "I've lost my glasses, but I can't look for them until I find them." If you would do certain things, you would be less depressed, but you don't feel like doing them because you're depressed.
I usually prefer to perform some psychological assessment within a few weeks of initiating treatment, so we can determine if the issues mentioned above are applicable to the patient's situation and condition. Such assessments can help us decide which of several approaches is most likely to be effective at reducing the depression most quickly. Approaches that are often most effective include cognitive therapy (working to help change the thoughts and interpretations which are made automatically, so that depressed feelings are triggered less), a variety of therapeutic exercises such as writing a letter (not to send, just to express long-suppressed feelings) to a person who is connected to the depressed feelings, and possibly the use of medication if the depression is significant, and especially if it is related to additional mental health issues.
That said, I have had patients whose depression resolves in a month or less, and some for whom it has taken months or even years. Each case is different and there is no single answer that applies to everyone. But, the sooner you start the process, the sooner you will discover what your particular issues may be, and the sooner you may be able to resolve your depression and move on to a better way of moving through life. I often say that "your life should be the anti-depressant," but getting to the point where your life is engaging, challenging (in good ways), fulfilling and satisfying takes some effort and attention, and may require a therapeutic process to "kick-start" it.