How long can periods go for ive had my for three weeks now?

Longer sometimes: If you have had hormonal changes such as stoping birth control pills or a miscarriage you can have prolonged bleeding, however you can also become anaemic from blood loss so if your vaginal bleeding does not resume a normal cycle by the time of your next perion you should seek medical attention. Sometimes you need a short course of progesterone to restart your cycles, if that does not work should