How long do periods last?
A menstrual cycle is normally between 24 and 38 days, but this may vary from cycle to cycle and person to person.
The first section of your cycle is the period, and this happens when your body realises you aren’t pregnant and the lining of your uterine lining is shed.
At this point, your oestrogen and progesterone levels are low, so you may experience weight gain, a decreased sex drive, mood swings, depression, cramps, lower back pain, headaches, and other symptoms.
Periods can last between three and eight days, but for most people they last five days.
source: express.co.uk