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.

Illustration of a girl looking inside her skirt to find if her periods have arrived, to her side are four panels showing the amount of blood flow in first four days of menstruation - Menstrupedia
References:
  1. Menstruation and the menstrual cycle fact sheet, http://womenshealth.gov
  2. Menstrual cycle: What's normal, what's not, http://www.mayoclinic.com