Should a woman abstain from prayer and fasting for 13 days, which is the current duration of menstruation due to a uterine tumor, or should she suffice with the previous duration (6 days)?

The default is that the blood a woman sees is menstrual blood and not istihada (non-menstrual bleeding), because istihada blood is due to an ailment, and the default is the absence of ailments. The maximum duration of menstruation is fifteen days. So, as long as the blood does not exceed this period, all of it is considered menstruation. Therefore, a woman must consider all of that period as menstruation, and she should not perform ghusl (definition) (ritual purification bath) until after it has ended, and she should abstain from what a menstruating woman abstains from.