I believe you can fall in love as quickly as the both of you give yourselves permission to.
I am absolutley sure people fall in love at first sight and stay that way forever.
I know for me its usually lust at first sight and then you learn more about them later and decide if you like them or not...and vice versa.
For me love and trust are closely related and can't have one without the other.
Good luck if you have fallen, be sure the other one feels the same.
Love is a most wonderful thing!