There's really no set time or age that a girl will get her period as every girl is different and develops at different ages.
Most girls get their periods by age 12 to 15 years old and if the girl hasn't gotten her period by 16 or 17 then I would say that the girl is late on her period.
However it could just be that the girl is developing differently.
Some girls get their periods as early as 10 and some as late as 15 years old and if you've gotten your period by age 15 then you're developing properly.
If you're concerned about your health and haven't gotten your period by age 15 then you might want to see a doctor about it but usually nothing is wrong with you.