Hazrat Ibn Musayyib and Ibrahim state that the waiting period (istibra) for a slave woman who menstruates is two menstrual cycles.

17482 - From Abu Ma'shar (1) from Ibrahim (they said) (2): The slave woman is to observe two menstrual cycles for istibra’ if she menstruates.

Hazrat Ata says that a slave girl was brought to me whose istibra’ (waiting period) was being observed; after one menstrual cycle, her pregnancy became apparent. After that, they began to observe istibra’ through two menstrual cycles.