It is narrated from Tawus, may Allah have mercy on him, that if a slave girl does not menstruate, then her waiting period for Istibra is forty-five (45) days.
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1211
It is narrated from Imam Awza'i, may Allah have mercy on him, that I asked Imam Zuhri, may Allah have mercy on him, if a man buys a minor slave girl who has neither menstruated nor is capable of becoming pregnant like those of her age, what will be the duration of her 'istibra' (waiting period)? He replied: Three months.
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1213
It is narrated from Yahya bin Bishr, Ikrimah (may Allah have mercy on him) said: (Its duration) is one month. Imam Darimi (may Allah have mercy on him) was asked: What is your opinion? He replied: Three months is a precautionary strong duration, and one month is also sufficient.
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1215