How long does it take for low feelings/moods to turn into depression?

They might not: Low feelings/moods do not always have to develop into the full syndrome of depression. Depression involves mental, emotional, and physical symptoms, lasts 2 weeks or longer, and may not be responsive to usual methods of "cheering up." simple low mood may lift with changes in circumstance. A great resource of info about depression: http://www.Nimh.Nih.Gov/health/topics/depression/index.Shtml.