I know people who have married after 3 months of knowing each other and it works, and I imagine the same can happen for founders -- but -- who wants to take that chance? The ideal "marriage" whether in relationships or business settings (IMHO) is one where you observe the person before you are tied to them (e.g., before they are on their best behavior due to courtship), and ideally for a longer period of time so you can see how they are in stressed situations.
Everyone is usually on their best behavior during dating, so you don't know how people react under real pressure (e.g., when millions of dollars of theoretical paper equity are on the line on some crucial decision, or when interests diverge.)