How many days are in between each fertile period?

Usually 24-25 days: Almost all natural pregnancies result from intercourse occurring within the 5 days ending on the day of ovulation (release of an egg). In a 28-day cycle, ovulation usually occurs on or about day 14 (day 1 is the first day of the period), so the fertile interval spans approximately days 10 to 14. The next menstrual period begins about 14 days after ovulation + 10 days to the next fertile interval.

About 21 days: In a normal 28 day menstrual cycle, the woman ovulates on about day 14. Healthy sperm can live inside the genital track for 4-5 days and perhaps longer. Following ovulation, conception is probably limited for 1-2 days. This means that from days 9-16, a woman is potentially fertile. Now, getting to the question. From day 17 today 9 of the next cycle, there are 21 days. This can certainly vary.