Interesting and honest question
I'm a professional fashion and advertising photographer, that means I earn a living from my shots and I'm recognized, write masterclasses, run big workshops overseas - I don't have a day job.
If you're starting out, still have a day job and beginning to earn regular income through photography...it's up to you how you want to label yourself, you could be ballsy and state that you're a pro or call yourself semi pro. I'm all for confidence and I think if you're earning a substantial amount regularly and aim to grow further then shoot from the hip - call yourself pro.
Of course, once you give up your day job then there's no question about it