Choo I think you can feel completely smitten very early on, once you met someone, it's natural if you like someone, they like you, the chemistry is there and so is the sexual tension, and if it's all mutual then you want to be together.
I think what everyone on this thread is warning about, is that however wonderful these feeling are, you only ever really know someone over a period of time and that it's always worth having that knowledge in the back of your mind to balance out the "high". Enjoy all the fabulousness of feeling smitten but don't go planning a happy ever after until you have known someone a while.
Although, in my opinion also and to contradict myself above, i thought I knew my STBXH inside out and he turned out to be the most duplicitous, cheating, self serving bastard ever.
I didn't follow the rule book at all with Mr EA, from when I first messaged him, was totally emotionally invested before we met and here we are 6 months later, so there are always exceptions to every rule!