I got mine back at 3 mos. with my first and 7 mos. with my second (while tandem nursing). This time around, only nursing one baby who basically sleeps through the night, I expect to get it back soon again. I thought I might be getting it about a week ago because I had some bright red blood but it was only a little bit and only for two days, so I think it was just leftover PP bleeding, but I'm not positive.
The Mirena IUD is okay for breastfeeding and it makes a lot of women stop getting periods. I've considered getting one myself for that very reason (my husband got a vasectomy recently so I don't need it for birth control!)