Step one is the physical attraction and then step two is to work out compatibility. If yes to step one and step two, no reason not to fall in love. Without setting a time limit, I think you should know whether you love someone within the first 3 to 6 months. The harder part is to be really sure the other person is in love with you whilst always remembering that you can't make someone love you.
I've known love at first sight it actually was someone who caught my interest with his look of love. My ciriousity drew me back for more, their kindness to all spoke volumes to me. You can take it from there.