The doctor that put it in said i could have sex that same day, and i did, with no protection, and i didn't get prego. :-) But you do have to go back in a month and have it checked because very rarely they will fall out(my doctor said i would notice it if it had) or even rarer the will migrate up further then that are supposed to be.
Thanks for your replies. This is what I found on the new zeland version of the mirena site (why do they have different q/a section for different countries?).
How long should I wait to have sexual intercourse after the insertion?
To give your body a rest, it is best to wait about 24 hours after having MIRENA inserted before having sexual intercourse. However, as soon as it is inserted, MIRENA will prevent pregnancy.