Namahottie, as always, hit it right on.
Of course, the time period of symptoms has to be weighed against the severity.
For example, if you've been feeling mildly blue for several months, although you function well, you might want to seek help; just going to talk about it, maybe get some meds, etc. can help you feel better immediately sometimes. Also, time to get out and socialize more, and get exercise.
If you've been feeling very down for two weeks or more, but still function "ok" in your social and work life, get help promptly.
If you've been feeling like you don't know how you're gonna make it until tomorrow, get help NOW, and if they tell you there's a waiting list, tell them you need to see a doctor NOW. They should make room or refer you to someone else.