You should go through at least 1 full pack of pills and have your period before having unprotected sex while on the pill. It is still possible to ovulate in the first 4 weeks of your first pack. it takes time for some women's bodies to adjust and so it's a good rule of thumb to get through 1 whole pack have your period and then once you start a new pack then if you want have unprotected sex. But please only do this if your in a monogamous relationship and are just wanting to prevent p/g as the pill doesn't stop STD's.