How Long Does Meprate Tablets Take To Normalise The Periods?

I have problem of irregular periods. periods get delayed 5 to 6 days everytime, however this time it is almost 20 days and i still not get periods. i consulted a Dcotor and she prscribed me Meprate tablets so my question is when it will work as its 3rday i m consuming tablets and stillno results.