In most cases, you've seen each other and talked to some degree (unless a blind date). When I dated, I pretty much knew in about 10 minutes if I'd see them again or not. If there was a nice chemistry and connection, a strong physical attraction, what's the difference? It was already past a one time thing (meaning we both knew we wanted to and would see each other again, usually talking about it).
1st date, 3rd; one week, 2 weeks, I never placed any significance on it. To me, if we end up in bed after 2 weeks, or the first weekend, it was more of an attraction/comfort/chemistry thing, not a priority.
I guess I ~knew~ what my intentions were, and am fortunate to have the ability to read people on the inside incredibly well. Most people probably aren't very intuitive, so can be drawn in to the wrong person, for the wrong reasons. I can only say from my perspective as a man, if we by chance ended up spending the weekend together, it would be much more of a good thing for the long term than not.
I realize that's probably in the minority and I probably didn't explain it that well :-P