I had my first Mirena coil fitted 7 years ago and was told that it would stop me having periods. The doc did say that it could take a while to settle down and I could experience some bleeding for quite a while. What an understatement!
I had what seemed like a heavy period for 4 months! It eventually tapered off and I didn't bleed again. I had to have the coil replaced 2 years ago and I was dreading going through the same thing but I only had a slight bleed for 1 day. I've a couple of friends who've had this coil fitted and their heavy bleeding has lasted from a month up to six months. It is normal, but if you are worried or feel you are becoming anaemic, seek medical advice.
Good luck.