That's an impossible question to answer. You need to know as much as is required by the application you want to create. Someone might have lots of knowledge spread of numerous topics and not have enough to create a particular app in a particular area. Even those of us who have been doing this for years don't always know enough to create the application we want when we start out. You generally identify the areas where you're deficient as you go and learn what you need to on demand. You should certainly have a decent grounding in the basics first, e.g. how to write loops and decision statements but there's no specific line of knowledge that you must cross before being about to write a useful application. No matter where you are when you write that first useful application, I guarantee that you'll look back on it later and face-palm at how bad it was. I've been doing this for over 10 years and I look at stuff I did a year or two ago and I still see things that make me wonder what I was thinking at the time.
