How soon is too soon to talk about kids?

I've been talking to a guy for about a month and we are obviously interested in each other and want to pursue dating, but we are currently long distance over the summer so we are just waiting to see each other again in person to actually start something. However, we sort of briefly touched based on an important topic and that's marriage/kids. We haven't really spoken about marriage, but he's aware that I only date seriously. And we do plan on having these convos. But when is too soon to talk about kids?