And by this, I mean a girl you are interested in for more than just sex or a one night stand.

I feel like I have a very distorted view on this and I would like to hear some opinions.

In high school, I spent three years hopelessly crushing on one girl until somehow I ended up dating her. Conversely, my next "relationship" of sorts happened in college when I met a girl who I was immediately interested in, and it was clearly reciprocated. Within 48 hours of meeting her, she was in my bed.

I feel like these two situations are two extremes, and I don't really know where the more "normal" middle ground is. Obviously the first situation is horrible and I vow to never repeat that, and the second situation is an ideal that rarely happens.

Because of my experience, I have no idea how much effort is ok to put into a single girl before consciously deciding to stop trying and move on. I know that obviously some girls will be interested in me right off the bat, and some girls may not feel that way at first but can be swayed. And obviously some girls just will not be interested in me no matter what. I guess another way to phrase my question would be, how can I tell if the girl can possibly be swayed, and at what point do I know that I don't have any chance?