I think people fall in "love" three or four times during a relationship. Although obviously all relationships are different, so this is nothing but a generalisation.
The first is when you start getting to know them, and find them wonderful, or interesting, and just want to know as much about them as you can, and find an attraction between you.
The second is when you have been dating for six months or so, have got to know the person relatively well, and really respect who they are, enjoy being with them, and know that you want this person to be in your life for longer. This is when people normally say "I love you" from what I can tell.
The third is when you have been together for a period of years, probably lived with them, and know them very well. When your lives are intertwined to a degree and you feel like your life is better for it. You would do almost anything to make them happy, and you know they would do the same. Where you wouldn't ask anything hurtful of them, and they wouldn't either. At the moment where you think "I want to spend the rest of my life with this person in it" is the third and final time (in this shit analogy) that people fall in love. To me, this is real love.
I was certain that I loved my DP after a week, but looking back I barely knew him at the time.