I've heard that there's two opposing views on this - on the one hand, you don't want to introduce your kids to everybody that you go on a couple dates with, that's crazy making. on the other hand, you don't want to get emotionally involved with someone that a, doesn't get along with your kids, or b, your kids don't like.
My current boyfriend met my son about a week after we started dating - it was important to see how they got along. This was partly because I knew right away that this wasn't going to be a short term fling and I needed to make sure that he could get along with my son and that my son liked him.

I introduce my kids early in, but I introduce the men as friends. I leave it that way too, nothing goes on in front of my kids even though I have them all the time. I also make sure that my kids and my partners do not have a close relationship. I did that once and my son cried longer then I did when he cheated on me, and we ended it. So also because how close they are getting.

I normally wait 6 months to a year.