When do you know that the guy is Mr Maybe Right, somebody you'd like to date? After a month of friendship? After hooking up at a party? After a one-night stand?
Scientists tell us, that actually you can tell within the first minute, a fraction of a minute maybe. The chemistry is either there, or it's not. That's why speed dating is so popular: instead of spending a whole dinner date with somebody you may or may not fancy, you get to meet ten potential date candidates.
You may have a list of questions to ask them, you might even have a list of requirements in mind. No matter. If the attraction is there, you'll know.
Now, if only there were a research paper to tell us whether a given relationship will last....