Verified answer

7 days on average

it is supposed to last from 4 days to 8. A week is the usual amount of time.

http://www.womenshealth.gov/faq/menstruation.cfm

3 to 7 days is about "normal"

Please talk to a trusted adult if you feel you are in trouble.

If you have to write a note and just hand it to your parent or adult then that's fine too. But, don't feel scared and upset. They can help you. Good luck =)