ANDY'S ANSWER: When you first start getting periods, you will probably notice a small amount of blood only for a day or two. As they start arriving more regularly, periods that last between 3-5 days is normal. Even a heavy period will not usually last for more than 7 days.
Periods can very irregular for the first few years anyway, so saying it will come every 28 days is really inaccurate. Even once you start getting a regular period, it could vary by several days. Your cycle could be longer or shorter than a friend's cycle. Some girls have periods within 21 or 22 days and other's periods are spaced as many as 34 or 35 days apart. So in reality, 28 days is just an average, and it is rare to have a period that is spaced exactly the same number of days apart each month anyway.
Many women get signals from their body in the days prior to their period's arrival. Pay attention to how your body is feeling and then keep track of the dates on the calendar when your period does come. You will soon be able to figure out your own general menstrual schedule.