How can the duration of menstruation be determined if the questioner had previously miscalculated the days of her period, and are yellow and turbid discharges considered menstruation after the six days have passed, especially since she had considered them part of menstruation since puberty?

Scholars differ regarding the establishment of a woman's customary period, and the most preponderant opinion is that it is established by a single occurrence. Yellowish or brownish discharge (Ṣufrah and Kudrah) is considered menstruation if a woman sees it during her customary period, or if it immediately follows blood discharge, even if outside her usual days. So, if your customary period is six days and you see blood for five days, then see yellowish or brownish discharge on the sixth day, it is considered menstruation. The same applies if it immediately follows blood discharge, even if it exceeds the customary period. Other than these cases, yellowish or brownish discharge is not considered menstruation. And for one who cannot act upon this preponderant opinion, it is permissible to adopt the view of the majority, who consider yellowish and brownish discharge to be menstruation during the time of possibility, absolutely.