When I started modeling for art students, one of the instructors started referring to me as a "professional model" as soon as he found out I had modeled for artists outside of the classroom setting. I didn't start considering myself a "professional" until I decided to put together a portfolio; that's when I had visual evidence of the number of classes/groups I had worked with, & the wide array of representation that had been done with me as the subject.I even got a few letters from artists I had worked with, proving that yes, they did consider me a professional long before I had done so myself.
Writing is a little different, but it's the same concept because it's another art form. Once the writer is content enough with the publishable work he or she has put out, when the quality &/or quantity meets whatever subjective standards the author has (with the realization that those standards can change without a moment's notice) a person can consider him- or herself a professional writer.
Since earning my Bachelors degree in 1995, I've been calling myself a writer, albeit a non-practicing one. Now that I've started on a major project, doing well at it and seeing ways to improve my work, I feel comfortable leaving out the "non-practicing" part of that description. Whether my work is "professional" or not, well, I'll leave that determination for other people. Thought-provoking question!