Praise be to Allah.
The following question was presented to Shaykh Muhammad ibn Saalih al-‘Uthaymeen:
A woman is asking a question; she says that she has become Muslim but she had a haraam relationship with a man. Should she wait (observe ‘iddah) before she gets married, or not?
He replied, may Allaah preserve him:
One menstrual cycle is sufficient. And Allaah knows best.