Depends on how long I've been with my partner. If it's really serious, then at least a year. If things are still new, only a few weeks to a few months ahead.
When I was 19 I had a girlfriend launch into this huge discussion not even a week after we got together about her plans to get married and have kids ASAP. Needless to say, we didn't last very long, so I try not to do that to other people.