Hi there
I'm 21 years old, but I had the same problem when I was her age. My gyno put me on birth control to regulate my hormones and it did wonders for me. However, now that I've been on the pill for so long, it isn't regulating my cycles anymore no matter which pill we try. Some women's bodies just don't have the capability to regulate themselves. Hopefully this is just an issue she's having because she's going through puberty, but watch it closely because it could be a problem she always struggles with. Some people think women from my generation and younger are struggling with this because of all the hormones injected into our chicken and milk.
I would take her to a MD to make sure she is not anemic.
But some girls will have major hormonal surges that will cause prolonged bleeding. My DD (13) had a cycle of 10 days on 5 days off for a year. She has since balanced out and has normal periods.
The main thing is to make sure that your daughter has no structural or blood issues.
That's too long. Take her to a doctor.
i would take her to a gyn