How long does mefenamic acid take to work?

my gp has had me on the pil for years due to endometriosis i recently started having breakthroug bleeding and new gp said i was to have a break from pill i was concerned as no period for years and he felt i had cervical erosions so anyway i came off pill and as predicted i cant move and blood is ridiculous as well as pain beyond words!!! another gp prescribed me mefenamic acid to stop bleeding but how long will this take am due on niteshift tonight!!