Well, again, this depends upon the person, but on average if the person is not suicidal or self-harming themselves, then studies have shown that depression usually goes away within six months, if left untreated. Probably not which is why asking for help is the best bet. With help, depression heals much faster. If a person is suicidal, then if depression is left untreated, it can really lead to suicide. Studies have shown that the majority of the population will experience depression at least once in their lives.
Depression comes from life.
Though a wonderful gift, life does have it's challenging, difficult moments and because we're human, it's natural for us to be affected by those events. Hopefully, these answers have given you a little more insight into depression. With knowing that it's an illness that can, and definitely should, be treated, I hope it will encourage you to reach out to your school counselor, your youth minister, parent, teacher or other trusted adult for help. You are worthy of life and you are loved.