The best way I can describe it is they have to "probe" your uterus before the actual insertion to make sure they know how deep, wide, ect. it is. This ensures they get the IUD in the correct location so that it can work properly. They put you in "pap pose" in the stirrups with the speculum in, and then the doctor uses a small metal object to do the "sounding". I didn't ask for details, so I can't say exactly what was going on down there from a medical standpoint, but it felt like she pushed the probe all the way back until she felt the top of my uterus, and then did the same with the sides. She told me it is not uncommon for women to have cramping during that portion. After the doctor is satisfied that they know where to put it, they slide a very small plastic tube inside (it reminded me of a straw). The IUD is in this tube, and when they remove the tube, the IUD remains in your uterus. Then trim the strings to an appropriate length - I think every doctor must do a different length, because when I had it removed, it was by another doctor in the practice and he asked the nurse to look through my chart to see if the first doctor made a note about how long they were cut.
I will say that in my case, I have a tipped uterus, and the doctor seemed surprised about how deep it was. I love my doctor, and have been a patient for over 7 years, so I trust her and trust that she knows my body. It seemed like she took extra time to make sure everything was where it should be. I don't know if any of these things were a factor in the amount of pain I experienced. I also don't know if maybe where you are in your cycle might be a factor - open, soft cervix vs. closed, firm cervix. It's worth asking about. I can't remember where I was in mine.
Honestly, I wasn't traumatized by the actual insertion. I would say an hour is a fair estimate of how long it took from beginning to end, maybe a little shorter. When I got home, I told my husband that at least it was over, and I wouldn't have to think about birth control again for at least another 5 years. When my doctor inserted the Mirena, she told me that the FDA is actually considering extending the time it can be used to 7 years, which would be great. What really surprised me was how much it bothered me to have this thing in me. I never expected that. Never. I was so sure this was what I wanted.
There are so many birth control options out there at every end of the spectrum, from natural family planning to sterilization and a million in between. An IUD is a pretty big commitment, especially if you are paying out of pocket (I paid somewhere around $500 for the IUD, and about $200 for the insertion and recheck appointments). If you want something you don't have to think about everyday, there's the patch or nuvaring. If you want something non-hormonal there's temping or barriers (diaphragms/condoms/sponge.) You have a lot of options. I'd make some pro/con lists and share them with your doctor. I don't want to scare you, because many women have had great experiences with an IUD. You're going to find people who hated it. But the same can be said about all birth control methods. Do some more research and don't make a decision until you're ready.