Yes it can happen Clinical depression can be without any reason due to neuro chemical imbalances It takes minimum 15 days

Hi thank you for asking two excellent questions. Treatment of depression includes medicine and psychological therapy (Cognitive behavioural therapy). If one is only taking tablet they may improve in 5-6 weeks time. Working on self help approaches will help to maintain improvement. But it is advisable not to stop medication abruptly. Depressive disorders are complex. Sometimes causes may not be apparent. However having in-depth consultation with a Psychiatrist will help to find the risk factors. This is important to prevent future depressive episode. If appropriately treated recovery will be good. Best wishes

Anti-depressants generally start showing their effect in a week but for the full effect to show it may take 4-6 weeks. However, there can be significant variation based on the anti-depressant and the type and severity of depression. Depression can happen without any apparent cause. There are two broad types of depression, one due to stressful life events and other without them.