My husband and I adopted both our children. I assume yours was a "matched" adoption otherwise it would be obvious that your daughter was adopted. Ours was also a "matched" adoption in that our children look like they are our biological children.
We have told our children about their adoption since they were tiny babies. Telling them their stories was just one of many bedtime stories they were told. In the beginning it was more for us than for them - for us to get used to the words we wanted to use. For example, I thought that I would call their birth mother their tummy mummy but when I started telling them their stories I found I wasn't comfortable using that term. We also read them adoption story books along with all their other story books. We socialise with other familes formed through adoption. So adoption is part of their lives, something they have always known and they know other similar families. It's nothing unusual for our children. It's something they can talk about and ask about when they want to, which isn't very often, although they do love hearing their stories.
My advice would be to tell your daughter as soon as possible. Otherwise you run the risk of her finding out from someone else and there could be very real trust issues. Some children who are only told later or who find out by accident feel betrayed by their parents and find it difficult to trust their parents. Along the lines of, "If they lied to me (even if the "lie" was by ommission) about this, what else have they lied to me about?" I would not wait until she is 12 and starting to go through teenagey-type identity issues. Those teenage years can be difficult for children who were adopted so better to have let her processed the fact that she was adopted before then. The longer you keep her story a secret from her the more chance there is of her feeling like it something to be ashamed of when nothing could be further from the truth. Adoption is just another way to have a family and your daughter needs to know that. She needs to know that she is special because you chose her to be your child and you are her parents in every way that matters - you love her and will always be her parents.
There is a wonderful website called
www.adoptivefamilies.com that gives advice on Talking about adoption and has forums on everything to do with adoption. Good luck! I hope the telling goes well and that you can be open with your daughter and answer all of her questions as honestly and age-appropriately as possible.