At what point did Sophie and Alberto become real? When did they become more than a character in a book? Or were they always 'real', and they only needed the book to be written for things to happen and so that they could escape? We know that Sophie has memories from before the events of the book, but did those events actually happen or was she 'created' at that moment and the memories implanted by the major?
I believe that Sophie and Alberto became real as they crawled to the tip of the rabbit's fur. The more questioning they became, and the farther along they crawled, the more real they became. They no longer said just the words the major put in their mouths, and acted just as the major wished.
And this fits in with Alberto's view of philosophy - with the entire analogy of the white rabbit. Those who crawl towards the tips of the rabbit's fur are more alive, more aware, than those who just accept it. And at the end, they look into the Magician's eyes, and perhaps even leave the rabbit, and this is what Sophie and Alberto ended up doing.