My understanding is that it's different for everyone. I can tell you my experience (though you may not want to hear this!). I had off and on bleeding for 6 months after having mine put in! Yes, that's months!!! I kept asking my OB/GYN about the advertising that says there's some slight bleeding and then you may not get your period at all. She said that the fine print says you can bleed for up to 6 months, and it's only if you go longer than 6 months that they are concerned. It was horrible, and I can tell you that I had serious doubts about this being an optimal form of birth control. I did not however have any other health issues.
I've now had it for 3 years, and my periods are fairly light (if at all). So, after the initial nastiness, they finally settled into a better routine. In some months, I feel all the other symptoms of getting my period and then I don't have any bleeding. In others, they are light to moderate bleeding. So, all in all, after the looooonnnnngggg 6 months, it is now OK. I still don't know if I would do it again, since it was really inconvenient initially.