Astanga Hrudaya
मासि मासि रजः स्त्रीणां रसजं स्त्रवति त्र्यहम्। वत्सराद्वादशादूर्ध्वं याति पञ्चाशतः क्षयम्॥७॥
In women, the rajas (menstrual blood) which is the product of rasa (the first dhatu), flows out of the body for three days, every month, after the age of twelve years and undergoes diminution by the age of fifty years
english translation