Depends on the situation and your age. By a certain point of your life you can tell what qualities you want and which ones you can not, nor do not want deal with. When I was much younger I dated a girl for 6 years and never really knew if I wanted to spend the rest of my life with her.. Currently I have been dating one for 6 months and knew almost right away, now it is just a matter of finding a nice ring.
IMHO, long term relationships need to have multiple layers in order to sustain themselves. In the past I had the great sex but not the great conversations. Or great conversations and lifestyle but crappy sex.. In the end, when you find someone that entertains you, makes you happy, is someone you can relate to and likes to have sex as much you, the decision will be a no brainer.