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