What is the duration of menstruation that should be considered customary in the event of not remembering previous cycles and menstruation continuing for a long time, and is it necessary to perform ghusl more than once due to the continuation of menstruation and احتلام (nocturnal emission)?

The correct view is that a habit is established with a single occurrence, without requiring repetition. Therefore, the duration of blood discharge you see before istihadah (prolonged non-menstrual bleeding) is your habit. If you are unsure between nine and ten days, make it nine. Perform ghusl once after this period ends, then perform wudu for each prayer after its time begins, and pray as much as you wish with that wudu until the time expires. After that, you will have the rulings of ritually pure women until the same time comes in the following month, when you will cease praying for the duration of your habit. A single ghusl suffices for both menses and janabah (major ritual impurity) and removes both impurities.