“How long can depression last?”

Oh gosh. This is a complicated question. I would encourage you to talk to a therapist and see if you can identify the origin of your depression. A good therapist can help you identify aspects and themes related to your depressive symptoms. Some people with depression experience it in a long term and chronic way. Others find their depression is related to something specific in their lives and are able to make adjustments that provide relief. Additionally, of course, your brain chemistry could be involved and a trial of an antidepressant may be helpful in symptom reduction. I recommend you see a psychologist or therapist to help you identify the best course of action for you. Hope this helps.

It varies among people.

It depends on many issues. It's important to consider each person individually.

Depression can last for weeks, months, or even years if untreated, but with treatment, many people experience improvement within a few months. For more information, please contact your mental health provider or, if you are in Texas, schedule an appointment with Mid Cities Psychiatry.

Answer depends on the cause. If situational average 6 months.
If lifestyle related it lasts until lifestyle changes are made.

There are many variable that can impact during and progress of depression. The environment, emotional skeletal makeup imprints, health issues, acceptance, meds, etc.
The Key is a commitment to improvement .