It varies from girl to girl

Not long enough, for some of us. On average, a menstrual cycle lasts around 28 to 30 days, but because we’re all different, a cycle can range from 21 to 35 days. Days 1-7 is when the bleeding will happen so you'll need to use either a tampon or pad during these days. The flow will be heavier at the beginning and become much lighter towards the end of this stage. Then the cycle starts all over again.