Getting periods back to normal?

I had a mini period 2 or 3 times a month from April to my removal in July. After removal, I have had no bleeding despite the Dr.'s warning that my Mirena was actually helping the bleeding and it would likely get worse. Tomorrow will be 28 days since I bled last. I did a pregnancy test which was negative, and I bought ovulation sticks a couple of weeks ago just to try and catch an ovulation with no positives yet. Does anyone know how long I should wait for a period without getting concerned?