The post-marital waiting period (Iddah) after marital release (Khul’) is the same as divorce, thus three complete occurrences of menses or with childbirth if pregnant.
“And if a man divorces his wife with an irrevocable or revocable divorce, or if annulment takes place without divorce, then if she is a free woman and menstruates her Iddah will be three occurrences of menses.” – (Al-Hidayah, Chapter of Divorce)
Ibn Humam mentioned in the commentary:
“This ruling includes marital release, as opposed to those who said that the waiting period from marital release is a single occurrence of menses.” – (Fath al-Qadir, Chapter of Divorce)
– Answered by Shaykh Noorud-deen (05.01.2021)