ANDY'S ANSWER: The "average" is 28 days, but I don't know anyone who has periods exactly 28 days apart every time. I've heard of periods that are 20 days apart and periods that are 35 days apart. Also, it's rare to find a girl who has really regular periods that are the same number of days apart every time. Most girls keep track of when their period starts each month (day 1) and start counting the days from there. You'll get a pretty good idea of what is normal for you.