What is the ruling on making up for missed fasting days if the duration of menstruation varied from 6-7 days to 9 days, then to two to three weeks, and reached 15 days during Ramadan?

There is no minimum or maximum limit for menstruation. The determining factor is the flow of menstrual blood. The Almighty's saying, "And they ask you about menstruation. Say, 'It is a discomfort (or harm),'" indicates that the ruling on menstruation is tied to the presence of the known blood which is a discomfort. So, if it is present, it is menstruation. Based on this, your regular period consists of the days during which blood flows until you see purity, even if it exceeds fifteen days, unless the blood flow continues for the entire month or stops for only a day or two, in which case it would then be irregular bleeding (istihadah).