How often do a girl have period?

Depends: Usually girls start having their period when they are 11 or 12 years old. Their systems are immature so maybe until 18 they will have irregular cycles.

It varies: There are a lot of factors that can impact a woman's cycle. A normal, mature hormonal system would have a period every month on average (21-42 days).

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.