How long does it take for the depo-provera shot to be active?

About one week: One week if given after 5 days of the menstrual cycle. If given during the first 5 days of the mesntrual cycle then it works immediately. Would still recommend protection for about 1 week.

Its fast: If you are referring the the birth control shot, it is considered protective in 24 hours. However, the first shot should be given when you are on your period. So pregnancy risk should be nearly zero at that time. For more info go to: http://www.Acog.Org/~/media/for%20patients/faq159.Pdf?Dmc=1&ts=20120922t1830582752.