Effectiveness?

I posted earlier today in the IUD group. I had Mirena inserted today, I'm doing well. Little to no spotting and my cramps have vanished after taking three Ibuprofein. My question is; when will Mirena be effective for me and protect against pregnancy? My gynecologist wasn't very specific. My periods became irregular in March due to gall bladder removal, and I haven't had my period in a while, though I'm not pregnant. I've heard if it's inserted 7 days after period, it's immediately effective. But if it's inserted any other time during the cycle, to wait 7 days before having unprotected sex. Is this true?