Wanting to adopt stepdaughter?

We gained custody of my stepdaughter about 4 years ago. Custody was taken away from her mother by the courts. A year and a half ago my stepdaughter went to florida to live with her mother again. Two months ago she came back to us due to abuse from her mom. Since living with us again her mother has had no contact what so ever with her. I was wondering if anyone could tell me how long does there have to be no contact between child and birth parent before I could petition the court to adopt? Would my stepdaughter whom is 16 have any rights as to what she wants? I can tell you right now that bio mom would not voluntarily give up her rights as that she wouldn't want to "look bad" to all of her friends and family. NOt to mention if she knew that that is what we really wanted she would say no out of spite. If my stepdaughter talked to the court and said she wanted nothing to do with her mom anymore and wanted me to adopt would that have any bearing on the situation? Bio mom also has a daughter that lives with her dad and has not had any contact with her in almost 2 years. Anyone ever been in a similar situation that has actually achieved adoption? BTW we live in PA