My depression episodes have ranged from one week, to 4 years. It is normal for someone with depression to always be at a "low" and then get worse sometimes. But is also normal to have depression come and go, with periods of remission in between. It just depends on the individual and what they are going through and how their brains operate. For me, for a long time I was at a constant low, with it getting more severe every now and then. I was moderately to severely depressed most of the time for a period of 4 years but now my normal mood is about in between low and high. Sometimes I have depressed days but they aren't very severe.