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. Also, you'll get to recognize your body's signals that you are getting ready to have a period, like a feeling of being bloated, maybe a bit crampy or backache, etc. Some girls also notice emotional changes around their periods, like being more sensitive or irritable.
Remember, having a period is not a good excuse for being bad-tempered with other people, so being aware of your cycle and what happens to your emotional state is a good idea.