If we're just talking carving, it took me about a full season of weekend trips (about 12 trips) when I first learned to snowboard to carve like a seasoned rider. It also helped when one of my friend critiqued how I was carving and what I was doing wrong. What really progressed me though, was going to bigger mountains. I was never sure why, but each new, bigger mountain I went to gave me more confidence and I began to really carve double diamonds with more precise control.
If you want to know how to have better control, you need to understand the fundamentals and put them in practice WHILE you're boarding down the hill. When you're going up the lift think about what you did wrong and then observe how good boarders carve, and imagine yourself doing the same. The more you visualize what you should be doing, the faster you will learn. Remember, take your time. When you're on a trail, use the whole thing and go slow. Going slow allows you to be mindful of all of the things you need to be doing to carve well. When I started, I found that going slow and trying to strive for consistent "S's" (whether narrow or broad) actually strengthens your legs. A lot of beginners put too much weight on their back foot and it makes their tail wash out. You always want the majority of your weight on your front foot. Carving consistently and keeping your tail in line with your turns is great for your legs.