I bled pretty much all the time for the first 6 months. Its like I would stop for a week and then start right back up again. From what my doctor told me that is normal. But I didnt feel normal.
I am surprised they let you get one though, because as you said it is very uncommon for someone who has never had children to get an IUD - it isnt recommended. Because of this you may have more/less bleeding, I don't know. I had 2 children before I got mine.
But since you are here I suggest you do some research and decided if the Mirena is something you really want invading your body. It is truly unnatural and can reak havoc on your body. Ask anyone here.