Suggest Treatment For Heavy And Prolonged Periods

Hello. I am 41 years old, average weight and height. I naturally have very heavy and prolonged periods (my mother and sister both had this issue). I had been taking mercilon for many years which controlled my periods wonderfully but because of my age and lifestyle my doctor stopped precribing them. I had a nexplanon implant fitted 4 weeks ago. I understand that this will probably take a few months to settle but I started a period 12 days ago and I am still bleeding. My doctor has prescribed me norethisterone. Trouble is I really didnt ask her enough questions about it. So can I ask I start taking them 3 times a day, when can I expect the bleeding to stop? Also, how long do I have to keep taking them after the period has stopped? Many thanks, Sue