As time goes by...

I've always wondered how people decide how much time has to pass before they can do certain things.

After I have dated him for 6 months, we can have sex.
After the second date, we can kiss.
After breaking up with a guy, I need a year before I can date someone else.
I can only get engaged to a guy after dating him for 2 years.

How are these times determined? Is there a scientific study somewhere that says that people show their true colors and are more trust worthy after 6 months?

I definitely am for waiting till you are comfortable with someone before taking certain steps, but what if after the second date with someone, you STILL don't feel like kissing them? I find it kinda hard to understand this train of thought because my life seems to be VERY allergic to time tables and formulas. When I was 10, I was sure that I'd have kids by 25...because 25 was quite ancient to me and I couldn't think of why ANYONE wouldn't be completely DONE with having kids by then. At the age I'm at now, I'm not even sure that I can handle marriage talk less of having kids.

So, my question is...how do you decide how long it's gonna take (ahead of time) to get to the next step of a relationship? Is this based on past experiences, or is it just an arbitrary number? Is it a goal or...?

[Title taken from song "As time goes by" by Frank Sinatra]