Lea, i also have a friend who had the mirena and she bled heavily for 8 months , she would ahve a day or two here or there where it was just spotting but the rest of the time she was bleeding heavily, in the end she decided to have it removed as it caused 'her' more troubles then it was worth.
That's not to say that the same will happy with you, you might be lucky enough to not experience any of that - i hope so!

Goodluck darl