Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.070.

All statements dealing with this topic have no sound backing. If a woman has a customary length of time for her menses, she should according to it. Umm Salamah asked the Prophet about a woman with a prolonged flow of blood. He said, “She should look for the number of days and nights that she usually has her menses and the time of the month during which it occurs. Then she should leave the prayer (during those days, and then afterwards) perform ghusl, tie something around her vagina and pray.” (Related by “the five”, except for at-Tirmizhi.) If she has no customary period to go by, then she can try to distinguish between the different types of blood. This practice is based on the previously quoted hadith of Fatimah bint Abu Hubaish, which states that menstrual blood is distinguishable and well-known to women.

All scholars agree that there is no minimum or maximum time limit between two menstrual periods. Some say that the latter period is fifteen days, while others say it is three days.