I put my first bf out for 6 months. He sticked around 4 years after that, LOOL.
Current bf: around 3 months. I had the hots for him

. Looks like he sticked around for only 9 monhs prior to that, hihihihi.
I think it depends on a lot of things. If they have a good chemistry, if they knew eachother prior to dating, how much they see eachother during a week, how well they get to know one another... I think that lust being taken off the list, trust is really important also. In my book, at least.
So I think that there's no common denominator... just people, you know?