How long does severe hypoxia associated with premature baby last?

It varies. Near the end of the 2nd trimester, certain areas of the fetal brain are particularly susceptible to drops in oxygen/ low blood flow, which can be caused by maternal conditions that necessitate early delivery for the safety of the mother or because fetal well-being is found to be compromised. Ob's get those babies out asap & neonatologists try to prevent further hypoxia, but the die may be cast.