It shouldn't hurt at all if done right TK, a little discomfort as you get into it (or should that be "as they get into it"?) and maybe a little after, but hurting shouldn't be part of the equation.
Well kept, short nails are the most important thing to remember (unless using gloves) and not to push things, a little at a time, wait until its comfortable, then stick with moving around without going further until you're completely at ease with it, and go a little further.
If you reach a point where you physically feel you can't have any more in, but want to stretch for that little bit extra, wait until orgasm, and as you start, then go for it.
Thats how it worked for me anyway