How often can a girl have her period?

In general: That depends on the length of your menstrual cycle.

Average 28 days: On average a woman will have her period every 28 days or so, but some people have shorter or longer cycles. During the first two years of menstruation it is normal to have periods at irregular intervals, but after that they should become more predictable. An app can help track how often your period comes, and may even help predict when the next should arrive.