Interesting question! And I don't really know the answer... you know what you say about feeling you were pretending? When R was born, that's just how I felt. I used to describe it to people, that I felt like I had been given a loan-baby for a practical exam. Like any time someone would turn up, take her away and in her place award me a B+ in a Nannying GCSE! It didn't feel like I was pretending exactly, or playing - it was far too full on a serious for that - hence I guess why I felt it was like a practical exam!
I don't remember when that feeling stopped - but it was months, not weeks. I don't think anything big happened - I just got used to her being around I guess!
Even now though, I don't "feel" like a mother. But that's in the same way as I don't "feel" like a wife, or a responsible adult
It's the same feeling I got on my birthday "well blow me, I'm thirty-x, how can that be?"
Perhaps when she actually starts to call me Mummy, it will feel like it? (she can say mummy, and responds to 'where's mummy' correctly - but she doesn't actually call me mummy, iyswim)