With my boyfriend/exs, it's usually been about 1-2 years of knowing them before I get together with them. One was in a relationship so I stayed back when I met him, though I did like him. But that was in my teens.
I don't fancy people instantly any more, I've become a lot more picky. Even if I did fancy someone because of their looks/initial impression, I probably wouldn't act on it because I want to find people who want the same kind of relationship as me. It will just get me hurt if I'm looking for something long term and he tells me after a few months that he only wanted something casual. Or it would be awkward if I wanted something casual and he was looking for long term.
Dating people as soon as you meet them is just asking for drama, because you don't even know if they are seeing other people or would just cheat on you in the blink of an eye.