This probably varies by the person.
Some people might get a single episode, then get through it one way or another, and never have another one.
Others might have a lifelong tendency or vulnerability to it, and in that sense it might always be there in the background. I'm fairly certain I was more or less depressed (although not diagnosed) in my teenage years and early 20's. In my later 20's things were brighter, although I probably had a tendency for some amount of depression, especially during the darker winter months of the year (I live in the north). However, there were definitely times when I wouldn't have considered myself depressed. I'm not sure I'd even say it was "there" at those times, in the sense of being actively present in any way, even in a non-dominant way.
Later on, I got another deeper episode of depression that I'm still battling.
Yet other people might have it always there in some way, as you say.