Birth control pills do not stay in a person's system for an extend period of time; they leave within a few days. Most women's periods resume within four weeks of stopping the pill, according to Mayo Clinic.
It can take up to three months for some women to begin having periods again. It may take longer to resume normal periods if a woman had problems with regular periods before beginning the pill. Regardless, it is not a problem for her to try to conceive immediately after stopping the pill. While she may not conceive immediately, she does not put the baby at risk if she does, notes MedicineNet.