It is narrated from Abdullah bin Umar (may Allah be pleased with him) that the 'iddah of a woman who obtains khul' is like the 'iddah of a divorced woman.
Imam al-Bayhaqi (may Allah have mercy on him) said: This is the view of Sa'id bin al-Musayyab, Sulayman bin Yasar, al-Zuhri, al-Sha'bi and the majority (al-jama'ah).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15596