Hello! If you are trying to get pregnant, don't stress out yourself and always eat on time, because the hypothalamus (part of our brain) that produces hormone called GnRH which is sent to the pituitary gland won't allow it to send since the brain knows that the body can't handle pressures of housing a baby. So, rest well and eat healthy.
About your question, every women differ from each other. Some may experience short spell and some might took them long. It's even different month to month but i guess the time frame when the egg is released from the ovary to the time of menstruation is 14 days for most women.Try to check on net pregnancyplanet.net or celebritybb.blogspot.com. There's a lot of info there if you are trying to get pregnant. Good Luck to your girl! ↑
