"I wrote an amazing song on my first try" -Said no one ever.
The more I meet successful career songwriters the more I'm convinced of the fact that practice is an absolute must. It's the same with all skills; sports being a great example. Anyone can sit and write lyrics and chords, just as anyone can shoot a basketball. But to perform in such a way that takes notice of conscience observers, you must make decisions that are clearly more calculated than the average person. You must know when to make lyrical choices and when not to. Additionally, you have to be able to do it faster and more accurate than most others. How? Practice.
You have to make the wrong songwriting choice 100 times to acquire the ability to recognize an error (without outside correction) during a writing session. This is not to say you will always write perfectly on your first draft, but you will start seeing a dramatic improvement on the time it takes to complete a song. That's when your skills really start to show
Developing your songwriting skills is a numbers game. One song at a time.
So how often should you practice? Well, how often do Olympic athletes train? There's your answer. Get to it.