My personal experience is I started with 500 words which I can roughly finish around an hour. After learning that it was too weak, I went for 1000 words. But it was still not enough to satisfy readers. As I continued to improve, I settled between 1000 to 1200 words. After I got experience and had gotten used to it, I could now write 1500 words to 2000 words without interruption within an hour.