How long does it take after first injection of the depo-provera provera (medroxyprogesterone) to be affective to prevent pregnancy ?

Two weeks: You should use backup contraception for the first two weeks after receiving your first shot.

Depo Provera (medroxyprogesterone): If you get Depo-Provera shot during the 1st 5 days of your period – then you have immediate protection. “Quick start” is getting injection after 2 weeks w/o sex and a pregnancy test. Need back up protection for at least 7 days with this method.