Answer: it can take a long time.
Professional writers write for many reasons. Not the least of them is to make a living. There is a famous writing story about Charles Dickens. His publication, Household Words, had been running a serial, which was doing poorly. Feeling obliged to jump in to revive the fortunes of his publication, he wrote Great Expectations. Might we all be so full of genius.
The point is, sometimes, in pursuit of mortgage payments, and the like, the professional writer comes up with an idea (and it may well be a terrific idea), pitches it to a publisher, and is accepted. Then all he/she has to do is write it.
In my experience, when you write in that fashion, it is like a forced march. That is to say, you know you must get somewhere, you want to get somewhere, you are willing to get there, but the journey can be hard, even torturous. That said, if you work on your text long enough, hard enough (and if you have an editor patient enough)—you can get to the point where your work comes to life. I think it happens when you leave your basic plot idea and connect in an emotional way with your characters.
I’ve been on that lonely march. Oh, happy day, when it becomes a walk with people you know, and care, about.