What is the correct duration for menstruation: is it 15 days or 7 days, and is it permissible to consider yellow and brown discharge after 15 days as istihada (non-menstrual bleeding)?

The blood a woman sees during the period of possibility (of menstruation) is menses (hayd). Yellowish or brownish discharge (ṣufra and kudra) is considered menses if it occurs during the accustomed period or if it is connected to blood. If your regular period has settled at 15 days before the onset of irregular bleeding (istiḥāḍa) and the yellowish or brownish discharge continues, then your sitting out for this duration and counting it as menses is correct, and the duration you count as purity is the rest of the month.