For me I'd say it was around week 2/3 that I had that dip from my throat to the pit of my stomach, a sick feeling, it was love. I realised I loved them at that moment.
But I'd say the time I could really think about it and embrace the feelings was week 6. Finally I enjoyed watching them for hours, kissing their soft cheeks and smelling their little baby heads!
Please don't worry if it doesn't happen straight away that 'bond' the 'buzz' your maternal instinct is built into you and when it happens its magical!