I don't know about how long it took to feel I 'knew' my first baby, but it took about three months for me to get used to the way she looked, she took so much after her father's family and looked nothing like I expected, so I felt like I'd birthed a little alien for ages!
As for how long it takes, it really is different for everyone. Some people say that you should fall in love with your child the moment it is born, but it really does take time to get used to being a Mum, and it is extremely common for it to take months for a real love and bond to develop, so don't ever feel that you are abnormal or a 'bad mother' if it doesn't all come together immediately!

Kerrie