I too am in the same situation, ex left because I was pregnant, had very little contact, then none, then came back on the scene when DD was about 5yrs then disappeared again.
I agree with Pegs and think you really have to be as honest as possible about the situation. I have always been very open and honest with my DD without badmouthing ex and also admitting to my own faults as I believe anything else would come back to haunt me.
I contacted my ex just before DD's 10th birthday, sent him some pictures and a little letter telling him how wonderful DD is, he replied to say thank you and said that if DD would like to see him then he would see her. I discussed it with DD and she chose not to but have continually given her the option since. As far as DD is concerned my DH is her 'dad' as he provides for her, loves her and does 'dad' things with her, therefore she can't see any reason to contact her bio dad, she doesn't need him. This might change as she gets older and I will always support her in any decisions she makes.
As for when to bring the subject up, I found my DD brought it up herself around age 8, I then sat her down and talked with her and since then, if we are ever alone say driving in the car, i will occasionally bring up the topic and offer DD the option of contacting her BioDad. We have discussed it a little more lately as DD has been learning about genetics at school and this has led her to ask 'you have tiny feet, why are mine so big' 'where do I get my nose from' etc. and I'm glad I can answer her honestly without any awkwardness.
It's a really difficult situation to be in and I suppose you never know if you are doing the right thing but you will know your DD better than anyone else and you should do what you think is best for her.
Good luck :)