The amount of blood loss during each menstrual period is about:

Menstrual blood is predominantly aerial, with only 25% of the blood being of venous origin. It contains tissue debris, prostaglandins, and relatively large amounts of fibrinolysin from the endometrial tissue. The usual duration of the menstrual cycle is 3-5 days. The average amount of blood loss is 30 mL. Loss of more than 80 mL is abnormal. Ref: Alford C., Nurudeen S. (2013). Chapter 4. Physiology of Reproduction in Women. In A.H. DeCherney, L. Nathan, N. Laufer, A.S. Roman (Eds), CURRENT Diagnosis & Treatment: Obstetrics & Gynecology, 11e.