I hate to say it but I had quite bit of bleeding after I had my Mirena inserted, I would say for at least two months. Which really ticked me off since I got the Mirena because I was tired of the breakthrough bleeding I was getting from the mini pill. But it did eventually stop and then I just used to get a monthly period. Some people are lucky enough to have bleeding stop altogether.
Hopefully you're on the tail end of the bleeding. From the reading I did, it seemed that extended spotting was in the realm of normal. But if it seems off to you, call your ob/midwife.