Should one stop praying upon seeing a small amount of blood resembling wound blood two weeks before the menstrual period is due, or when the blood turns into the usual menstrual blood, knowing that the normal duration of the period is 8 days?

If the duration of seeing blood exceeds fifteen days, it is not considered menstruation for the entire period. Rather, you are considered to be experiencing istihadah (definition) (prolonged vaginal bleeding outside of menstruation). You must refer to your previous menstrual habit: whatever corresponds to it is menses, and whatever is beyond that is istihadah. During the days of istihadah, you must perform ablution for every prayer after its time has begun, while taking precautions. After that, all rulings applicable to ritually pure women apply to you.