On an average a period lasts between 3 to 5 days. It is normal for a girl to have irregular periods in the beginning.During this time, the duration of her periods may vary from as short as 2 days to as long as 7 days. It can take up to a year for periods to become regular.n an average a period lasts between 3 to 5 days. It is normal for a girl to have irregular periods in the beginning.