When does a girls period become regular?

There is no specific age or time when a womans menstrual cycles will regulate, some womens cycles never regulate in fact. Even going through your teens and into your early twenties you will have phases of months or years with regular cycles then out of nowhere as your body changes your cycles can be thrown off. Things such as stress and ill-health can cause irregular cycles too. At very least you shouldn't expect regular cycles until adulthood, early to mid twenties.