Inman it is not so long it lasts normally. There is no normally, everyone is an individual and everyone is different.
If you are not on antidpressants and you are feeling blue more than a couple of weeks, you need to speak to your doctor about treatment. If you have been on an antidepressant for 3-4 weeks and are not feeling any better, it would be best to let your doctor know as that you may need a medication adjustment. Different medications, different doses work different on different people.
Hope this helps answer your questions. I hope that you are feeling better soon.