when is the right time to speak to your children about their bilogical father

i need some desperate advice. i have a 7 year old daughter and has believed her step father is her real bilogical father since she just known him as the figure of dad since the age of 1. shes a real bright and sensitive  girl and today she was watching a movie where the adoptive mum t old her adoptive child (who was like 18) that she was not her biological mother and this guy got real upset and she was like how come she took this long to tell him. I was shocked and brought my heart into pieces. now her real biological father left me when i was 5months pregnant and he only tried to make contact with me a couple of times when she was like two months old and like ago. but since he used to make use of drugs i thought the best thing is to keep him away from her for her good sake. i always thought that the best thing is to tell her when shes grown up but today i came to realise that they might not be the best idea. but im really afraid i will confuse her big time.