I picked "when I started making a living..." - though I haven't yet, so it's wishful thinking. For now, I call myself a "hobby writer", mainly for semantic reasons. When people ask "So, what do you do?" they're asking what I do for a living, not what I do in my spare time, or on the side. My answer to that in the literal sense has to be "I'm a webmistress", because that's what pays the bills. My answer to "what do you like to do/what are your hobbies" includes writing.
Once I have published a few things (published by a recognizable company that people who aren't writers might recognize), I'll feel justified to say that I'm a webmistress and part-time writer. If someday, by some miraculous feat I actually get to quit my "day job" to write, then I will unequivocally call myself a Writer to anyone who asks. Ironically, they will probably follow up with the same question I get now about being a webmistress: "You make money doing that?"
My reasons are really just for clarity - and not wanting to misrepresent myself to others who may not really understand the distinctions that writers themselves make within the community.
Just my personal reasons - I have no problem with anyone who writes calling themselves a writer...but I have no problem calling myself a hobby writer either (I don't feel that it diminishes me in any way), since it's what I do in my spare time, and I don't make any money from it (yet). I'm quite literal in that way.