Is fasting during the fifteen days in which the bleeding continued valid, and how much of the Ramadan fast needs to be made up?

The duration of blood flow and the period of its cessation must be determined to make a judgment. If the blood does not exceed 15 days, followed by a period of purity of 13 days or more, then it is menstrual blood (hayd). If the duration of bleeding exceeds 15 days, or the period of purity is less than 13 days, then the blood is irregular bleeding (istihada). In this case, you should follow your usual menstrual pattern. Otherwise, differentiate (between menstrual and irregular blood), or consider six or seven days as menstruation.

If the blood was hayd, you must make up the days you fasted, because menstruation prevents the validity of fasting. If it was istihada, your fasting during the days of istihada is valid. If you omitted the obligatory ghusl (ritual bath), you must make up the prayers you performed without ghusl.