How long does it take to become a competent, professional freelancer?

Beginning Writer's Answer Bookedited by Jane Friedman

There’s no good answer to this question. Writers, even established ones, are continuously developing their skills, improving their style, and learning
more about their writing, their markets, and the world. In that sense, you’ll never stop striving to better yourself. A good test of your competence is whether your work is being accepted by publications or book publishers you respect. Being unpublished doesn’t necessarily mean you don’t have writing skills, but sales will probably make you feel more like a professional writer.