I think it takes a very long time to know. It's also possible to meet "the one" for a particular time in your life, but who may not be the one forever. I have friends who think every person they date is "the one" for the first few weeks-- but they've had probably 50 "ones" since I met them! I've had long-term relationships (5+ years) and while I thought I had met the one... I hadn't. We were just too young and grew in different directions. Personally, I would not consider even thinking about marriage until a year of dating-- and still probably not for 2-3 years. It truly takes that long to get to know somebody to see if you're growing in the same direction. I would also make sure that I wasn't fighting too often with my partner and that we wanted the same things (i.e. How many kids, how we'd divide up parenting duties with our careers, where we would spend holidays, where we would send kids for school, how much we would spend on a house, etc). I have learned from experience that having different ideas about those things can end even a really good relationship!