Good Evening,
There isn't unfortunately a specific time frame. It is not uncommon for doctors to try different mediations, dosages or combinations just because people respond differently to these. The doctor may try SSRI medication, SNRI, ND (norepenephrine dopamine) or other psychotropics.
What is usually helpful to some people is working with a therapist along the way. Some people are resistant as they believe that their depression is biological versus psychologically triggered, yet, a good therapist can be helpful to those suffering with depression.
It is important that you do not lose hope or become frustrated because of the slow progress or the lack of response you were hoping that the medications will lead to.
Having met and spoken to many individuals in this situation, it is not out of the question finding something that works.
One's own expectations and thoughts would be quite important because negative thoughts can cause changes in brain chemistry and the neuropathway to remain that in the past had had some contribution to depressive symptoms to remain. This is quite factual as it is the scenario where extreme psychological stress can cause damage to the frontal lobe of the brain that is hardly repairable and also when meditation practices had been shown to increase the level of serotonin in the body.
In a way, our response to things happening to us can help us or hinder us.
Licensed Psychotherapist
Licensed Psychotherapist, Certified Hypnotherapist, Self-Help books author