Gonadal precursors become sexually determined at around:
Human gonadal precursors derived from urogenital ridge remain bipotential until SRY expression around week seven post-conception, corresponding to 10.5-12.5 days in mouse model. Before this window sex cords morphologically identical and co-express low levels of both male and female program genes. Sry expression triggers Sertoli differentiation when threshold reached; without it ovarian program prevails. In humans determination window extends from week six to week seven when supporting lineage commits, Sertoli or granulosa specification begins, and hormone secretion initiates internal duct differentiation, after which fate becomes canalized.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 6: Gonadal sex determination at week 7.