Normally what happends is the first appt is you get the script, pregnancy test and swab to check for any signs of infection.
Then another appt to get the Mirena inserted. I was BF when I had mine in and it was also at my 6-8wk check I got things rolling.
There was no mention of timing only that she was pleased I hadn't had an AF since my initial bleeding after birth as she explained it would help with the settling in period.
Wear a liner to the appt for the after bleeding you may get. They will probably offer you one anyway.