What is the ruling regarding intermittent bleeding that lasted for 15 days, due to which the woman stopped praying, and then she learned that it was due to a "mass" and she is not married? Is this period considered menstruation, or is it istihada (prolonged non-menstrual bleeding) which necessitates ablution for every prayer?

The minimum duration of menstruation is one day and one night, and the maximum is fifteen days. If the spotting reaches one day and one night and does not exceed fifteen days, then it is menstruation. If it is less than one day and one night, or its duration exceeds fifteen days, then you are experiencing istihada (non-menstrual bleeding). In the case of istihada, you should act according to your previous habit, if you have one. Otherwise, you should distinguish between the strong and weak blood. If you cannot distinguish, then consider six or seven days of the month as menstruation, then perform ghusl (definition) and pray. Any bleeding beyond that is istihada, for which you must perform ablution for each prayer.