Reference: Fataawa Mu’aasirah – Page 86-87
Questioner: She apostasised and married a Christian. She then returned to the fold of Islaam and left him. So how long would her ‘iddah (waiting period) be in order for her to marry someone else?
Shaykh: Did she have intercourse with him (the Christian)?
Questioner: Yes, she had intercourse with him.
Shaykh: [It is sufficient for her to allow the passing of] one menstrual cycle according to the predominant opinion; And it is said that it is imperative she allows the passing of three menstrual cycles. However, the predominant opinion is [that she allows the passing of] one menstrual cycle. This is because her marriage to the Christian was correct (legal) whilst she was an apostate. And Allaah is the Most Knowledgeable.