If the tampon is inserted correctly AND you are relaxed, it shouldn't hurt at all.
If the tampon is correctly placed then you won't even feel that it is there! If it hasn't been pushed in far enough then you may get some irritation at the ******l opening.
If you are not relaxed then the muscles in the ******l opening will contract hence, there will be pain as the tampon tries to stretch the opening against muscular resistence.
It takes a little bit of practise to get it right but once you do, tampons provide a good alternative when pads aren't practical (e.g. when swimming or wearing tighter clothing).