Praise be to Allah.
We put this question to Shaykh Muhammad ibn Saalih al-‘Uthaymeen, may Allaah preserve him, who responded by asking:
“Did he have intercourse with her?”
(We said:), Yes, he had intercourse with her.
His response: (Her ‘iddah) is one menstrual cycle – according to the more correct view. It was also said that she should wait for three menstrual cycles, but the more correct view is that her ‘iddah is one menstrual cycle, because her marriage to that Christian was valid at the time when she was an apostate. And Allaah knows best.