My girl's case is worse. Sometimes around 7-8 days. Sometimes can be 10-15 days. Just clean for a week, period comes again. In the recent few months, period visits twice a month.
I asked doctor. She said some organs not yet developed and the period might not be punctual in the first few years. She said she couldn't prescribe "pills" to her as she's still too young.
I really hope her situation can be improved in the coming years.