Yes the maximum for Haydh is 10 days. However, since the blood is exceeding 10 days, then you look at the features of blood. So let’s say if 7 days the blood looks like Haydh blood (it’s dark and has somewhat of burning sensation to it), then you’d consider the 7 days as Haydh and the remaining as istehadha. Now if the blood looks pretty much the same all throughout, see what your sister’s period is (or one of your relatives who is close to you in age), and make your Haydh the same number has her number, and the remaining would be istehadha.
This is my understanding of his ruling. Yes, if you had a fixed period and the the following month this happens, you’d go by your own period of 7 days. But if the blood is exceeding 10 days for several months, then now you are no longer considered as having a fixed period.
If your sister’s period is the same as yours then you don’t have an issue. If it’s different then it’s best to do ihtiyaat (precaution) during the days that are different from her (if she is 6 and yours was 7 then do ihtiyaat on the seventh day) you would pray, but you also must avoid what a woman in haydh must avoid.