If it's not clearly friends with benefits, and you're not seeing other people, there's no reason to act all twitchy about calling it what it is. Doesn't mean you're about to get married, but you are a couple.
I think probably a few to 6 months, I know it seems like a long time however I need to know that I have a future with them and it's not just a fling and like each other but that we can and desire to be around each other often.
If she has a good personality, not long, maybe right away. If her personality is confusing I would wait a bit longer like maybe a week. If her personality is discomforting me I already know the answer, never.
It doesn't really work like that in my country. There is not even an actual translation for the word "dating". You're just in a relationship when you say so. The "get to know each other" phase isn't an official thing and many people make things official pretty quickly. With my boyfriend we decided we were in an official relationship while we were getting to know each other