It depends on what type of medication you are prescribed.
Antidepressants are taken daily and typically take 4 to 6 weeks for people to notice a difference. If you've been taking a medication and it doesn't seem to work, it may take additional time to make adjustments to your medication plan. Some of these adjustments including finding the best dosage of the medication you're trying, trying other medications, or even trying different combinations of medications.
Other names for drugs in this category include: Selective Serotonin Reuptake Inhibitors (SSRIs); Serotonin and Norepinephrine Reuptake Inhibitors (SNRIs); Norepinephrine and Dopamine Reuptake Inhibitors (NDRIs); Atypical antidepressants; Tricyclic antidepressants (TCAs); and Monoamine Oxidase Inhibitors (MAOIs).
Other types of drugs sometimes used to treat depression including mood stabilizers and atypical antipsychotics also take about 4 to 6 weeks for individuals to notice a difference.
If you've tried around three different medications without feeling better, you might have treatment resistant depression. Your doctor might talk to you about trying devised based treatments like electroconvulsive therapy (ECT), or transcranial magnetic stimulation (TMS). Some people feel better after one or two sessions of devise based treatments.
Drugs like ketamine are showing potential for quicker treatment of depression. These options are shown to have positive effects within one day and are currently being researched.