Menstruation of Hayd as defined by Islamic law "means the blood which is discharged from the female at given times pursuant to nature, without obvious cause. It is natural and not due to disease, wound, fall, or delivery."1
The time at which a woman starts her monthly cycles (i.e. periods) can vary greatly from individual to individual. The average age is in a woman's early teens but some have started as young as eleven. Again this is according to each person's biology. This is a biological process and everyone is different.
"Characteristics: the blood is red, thick and has a distinct, unpleasant odor."2
The age when a woman reaches menopause ( the end of her monthly cycles) is again based on the individuals biology. The average age is in a woman's fifties. Certain medical procedures such as a hysterectomy can cause its onset sooner. this procedure should only be done in the case of disease ( i.e. cancer, hemorrhaging, etc.)
The length of a woman's cycle can vary widely. The average is between 6 to 7 days. "Ibn Al-Madhir said: " There is no limit of days for the minimum or the maximum duration... "3
Rules Governing Menstruation
After finishing menstruating, a woman must perform ghusl (bath). This is a complete cleansing including the nostrils, mouth and head with water.
As to the condition of Nifas (postpartum bleeding), a maximum limit of 40 days was set but it is not absolute. I have read, if a woman is clear of bleeding before 40 days, she can resume her normal duties after the performance of ghusl.
Istihadah (non-menstrual flow): In the condition of Istihadah a woman must continue her regular duties. The rule of thumb in this regard is any bleeding, beyond a woman's normal length of cycle, is regard as Istihadah. For example, if a woman's cycle is 7 days, any thing beyond the eighth day would be Istihadah. Or a woman can follow the maximum number of days set by scholars. If a woman's cycle is normally short regard ten days as the maximum; a woman can use fifteen if her cycle is normally long. "If bleeding continues after that time, regard it as Istihadah, perform ghusl and resume normal duties."5 As long as it continues it is important to perform Wudu for each salat and use some protection (i.e. panty liner).